There is a difference between stereotypes and simply not fleshing a character out.

For example, your main character goes into a coffeeshop, orders, and sits down to read. At the next table are two women he works with. What are they talking about?

Mind you, you're not going to get into their heads. The narrator won't give background about them or tell their point of view. They may or may not speak to the MC (if they do, it is probably quite brief). The reader doesn't know their hopes and dreams, and doesn't care.

Some gender-based stereotypes could be that the women have secretarial/clerical positions, aren't really into the work, and their favorite conversation is gossip about other women they know or their husbands/boyfriends.

Some women (and some men!) have one or more of these characteristics, and that's okay. It's a stereotype when most of the women in your story fall into this or another trope, with or without some "girls who are not like the other girls." These aren't the only gender-based stereotypes. The women could be young, scheming professionals, who are gorgeous and fashionable. Just to name one of many possibilities.

Take a minute to think of them. In real life, what sort of women might sit together at a coffeehouse during lunch (or before or after work)? Of course, the possibilities here are endless. Choose any one of them. Maybe one is the owner of the company who is grooming her daughter to take over when she retires and they're talking about problems with a client. Perhaps they're analysts with cubicles next to each other and they're talking about taking their kids together to the upcoming county fair. Or they could work in the company cafeteria and go to the coffeehouse so they don't have to make their own coffee, thank you very much, and one is inviting the other to watch her race motorcycles that weekend.

It doesn't really matter what you choose because they're not important characters. They're background, like the coffeehouse. I didn't describe the place but you already know it's the type of coffeehouse with chairs and tables where people can spend time chatting or reading. Say I make a comment about the MC being glad that the pie of the day was peach, so he got a slice with his iced mocha. Now you know the place is local, not a chain, caters to the modern whim of different coffee choices (so not an old-fashioned diner), and, yes, it's probably late summer.

With characters it's the same thing. Choose a couple details and suddenly the reader has an image in her/his mind. (Whether it's the same image that's in your mind is not important.)

A way to do this with stereotypes is to make all the non-primary characters fit neatly into tropes and other expectations, then make the primary characters ones that don't fit the mold.

The better way is to see non-primary characters as the same diverse individuals people in real life are and throw in details to match. You don't need much for background characters, like the women at the next table. Secondary and tertiary characters will have names, descriptions, and roles in the MC's life. But they have their own lives and they don't just exist to further the MC. Write that in.

You don't need much visible fleshing out, IMO. It's all in the details.

Take the 'wild party friend', for an example, who swoops in and drags the MC into a crazy college party where everything goes.

At a certain moment, the MC tells the friend, already getting a bit beyond tipsy, they shouldn't be there. There's an exam coming up and... "shouldn't you worry about that, too?"

But the friend shruggs.

"Why bother? You have to relax and enjoy the fun while you can. You know, my neighbour keeled over because of a bad heart at 20. He didn't even know he had a bad heart. You need to carp the day, or whatever it is they say. Do you know what your problem is? You need another drink. Come on, I'll get you one."

That friend may never show up again, but that one dialogue line broke him out of the stereotype mold. At least a little.

But maybe you have a shopaholic as a tertiary character who bails out on the arranged shopping spree because her niece broke a leg and she was going to spend Saturday playing at tea parties and hairdressers to make her feel better. But hey, can they reschedule the shopping spree to Sunday? She doesn't want to miss the sales and shopping alone is no fun.

Sorry, the MC can't make it Sunday, the friends text back. Maybe next week?

Thanks for nothing, girls, seriously.

Or perhaps there's an important meeting where the MC is going to make a presentation, and as the other participants come in someone mentions that Jones won't be coming because his son is in hospital again. There's no need to give explanations.

Just drop a line that shows those tertiary characters have a life with priorities that are as important for them as MC's priorities are important for the story. They can start as a stereoype or not, but give the reader a glimpse of their own stakes.

One of the most interesting stories I read has a 'Romeo and Juliet' kind of story happening in the background. The MC sees those apparent 'extras', hears some unimportant gossip about them... and in the end learns they were involved in a love story that ended tragically. Can't recall the author or title, though, as I read it decades ago. Those two 'extras' had far more important stakes in their lives than the MC! And even if we never really met them, we ended up feeling for their tragedy.

I feel that Cyn and Sara Costa have explained how to avoid using stereotypes in the design of your story. My answer is to explore how to avoid using stereotypes in your prose itself:

Don't tell me someone fits a stereotype, let me draw that conclusion myself.

My uncle is your stereotypical depressed alcoholic has-been.
He was a total mess, sat in his chair.
He looked no better than when I last saw him a week ago.
He'd been watching the game earlier, obviously it had upset him.
He was shouting and raving about how it unfair all was, that he'd have done so much better if he was in there.

vs

My uncle sat, the smell of half a dozen cans of beer on his breath -
like always - wearing his old team jersey, which he'd had on the last
time I saw him a week ago, with the same mustard stain on it, too.
He'd been watching the game earlier, obviously it had upset him.
He was shouting, "I coulda done it! If I was in there, I'd have done it fine!"

Both of those take roughly the same amount of "space", but while one draws on a stereotype, the other focuses on the character themselves.

A reader may not know what a 'stereotypical depressed alcoholic has-been' looks like.
Maybe my vision of a particular stereotype is quite different to yours. Maybe it would summon the wrong mental image. They may have their place in culture, but stereotypes are an unreliable source of information at best.

The important part is to isolate what traits of a stereotype you want to use and use them. Maybe the uncle in my example is also a stereotypical opera singer and I want to mix that into his description as well? Suddenly the first description would feel odd, but the second description I could easily work in a mention of him having a large jaw and booming voice.

It must be said though, it depends on who your audience is. Maybe referring to a stereotype is the right thing for what you're writing, especially if you establish what that stereotype actually means beforehand.

How about a writing exercise?

Go to a public space made for hanging out and talking (coffee shop, park café, bar, you know what's around you) at three separate time frames (morning, lunch, afternoon, evening, late night, ...) once on a weekday, once on a weekend.

Now, sit down, relax, and... Eavesdrop! Just enough to get the themes of the conversations around you. Note them down, as well as a sketch-like description of the people talking (the kind of information you get at a passing glance). Compile all your observations, and voilà! You now have examples of non-stereotypical, but common, conversations that happen at that type of venue at that time of day.

You are trying to imitate life after all. So go observe it.

Interpret the secondary character for us

Stereotypes prevent your reader from doing mental work - you have to do that work for them if you don't use a stereotype.

Pick a point of view

A major difference between the main character and secondary characters is that you're not in the head of a secondary character. You have to use a different point of view, such as:

• The main character's impressions or observations of the secondary.


• The narrator's impressions or observations of the secondary.

This point of view is what will interpret the secondary character for your reader.

Pick a reason this character is here

To flesh out a character strongly in a small amount of space, you'll need something purposeful for them to do. Whether that's providing comic relief or acting as a way to tell your reader something about the scene, pick something specific that you want this character to communicate. A secondary character should affect either the actions of the main character or the mood of the reader.

The reason a character is there should make a strong impression on your chosen point of view.

Choose impactful actions

Since this is a secondary character, we only get to see what they do, not why they do it. The point of view you chose is going to observe what the secondary character does, how they do it, and provide a filter for the reader to interpret the why.

The why part is what stereotypes do so well. By using a ditzy party girl, we know how she's going to relate to the main character, what she might do, and most importantly why she might do it. You can make a non-typical secondary character appear understandable to the reader if the main character or narrator understands the secondary character.

Explicitly relate the reasons for a secondary character's actions and your chosen point of view's feelings.

Summary

Describe the secondary character and their actions from your chosen point of view, and make sure your chosen point of view relates to the secondary character in the way that you want your reader to.

Pick your stereotype. Use it. Break it.

Do you need a dizzy party girl? Great. She dresses like a dizzy party girl. You've heard her talk like a dizzy party girl. But you see her at lunch, alone, scribbling and thinking and scribbling; then she rolls her eyes and scratches a long line through it; just as her friend arrives.

"Hey, what are you working on?"

Brittney looked up. "Hey Gina. Nothing. I had an idea, but it won't work."

Gina spoke as she took her seat. "Well I'll turn that frown, upside down, because guess who came to see me yesterday?" She leaned in for emphasis, "Jeff Davis!"

"Oh my god!" Brittney said, grinning. "Serious? What did you tell him?"

The girls talk, they get up to leave, and you walk by the empty table, and spot the page of scribbles Brittney was first working on, with that final line drawn through it. All densely compact handwritten physics equations.

Now, you can say this is a trope, but it isn't a stereotype. Tropes are things that have been used multiple times in stories, but that doesn't make them instantly recognizable to the general public.

Stereotypes are easily recognizable, and the stereotype of the "dizzy party girl" does NOT include any expertise in the mathematics of quantum physics (or any other science); nor does the stereotype of the nerdy science girl include any elements of the frilly party girl.

So, even if you think my example above IS just switching to another trope; the prescription is the same: Be creative. Include a trait or characteristic that, in your own mind, just doesn't fit the stereotype you need for the role. In this case, being a party girl, and fashion girl, and gossipy girl, does not require being academically challenged or mathematically incompetent. Her choices for fun can be divorced from her professional choice.

IRL people are often born with natural talents, and they pursue those talents because they are fun (often fun because they are praised for being good at them), and that may become a profession or life-long hobby. But that talent doesn't have to be the only way they have fun; culture also determines that, and it isn't that difficult to separate the two things.

Pick your stereotype. Leave it recognizable, but break it. Take something central to the stereotype and invert it, or discard it, or add a disability -- She loves fashion, but she's color-blind, but also not embarrassed by that, because it is who she is.

Connect them to established elements

Even without stereotypes, you don't usually start with a blank slate. Characters don't exist in a vacuum, they navigate a - hopefully - rich and complex environment. Their relationship and/or reactions to another, more fleshed out character or setting element inform their personality and their place in the world or scene. If you want to characterize someone with as few words as possible, place them in a context they can boune off of and allow the reader to infer what you don't explicitly state.

A good exercise is to write just a single, realtively mundane line of description or dialog and insert it into a variety of existing scenes. What does it tell you about the character when they say this line at a party, in church, at a funeral, while paragliding, in bed, in a firefight?

The more detailed (and the more relevant to the line) the scene is, the more you can generally infer about the character. You connect a poorly fleshed out element to a detailed one and create a context that the reader can draw conclusions from.

This is essentially how stereotypes work, too: They reference an existing bundle of ideas the reader/listener is already familiar with (or will learn through repeated exposure).

The same thing works with any established setting element. Anything you flesh out and reference frequently will become a "stereotype" of its own. A very blatant example of that would be the four houses in Harry Potter, but also note how easy it is for the reader to think they have an intuitive understanding of, say, wands or brooms or mythical creatures.

I kind of feel bad writing an answer since everything I really want to say has already been said here. But it has been said by several different people in multiple answers and comments. So I kind of have to write a readable summary? Also for the things mentioned in comments, they are not written in the correct format. Even the answers are bit... Saying the right things but not necessarily in the right way to match the question?

Anyway, the key point to your question is that you want to be space efficient. The way to do this is to not actually supply a real description. Instead you supply the key points of the character and trust for the reader to fill in the blanks and generate the actual description. Many answers here do this exact thing in one specific form or another. You can use them as examples.

You say you have a good mental image of the characters. Use that. (Generally the first step would be to get that image but we can skip.) Make small itemized lists of the key points of the character that make them the person they are. One or two sentences. Note the relative importance of the traits and how they are connected to each other. They might be due to same background detail or one might have lead to another. These are also details you want to convey to the reader, not just the traits themselves. Otherwise they will fill in the blanks in some random way.

Priority would be based on how central they are to the character and story relevance. If it makes a difference for story, it is important even if it is a minor detail of the character.

Then think about how the traits express themselves in practice in small ways that can be observed in the context of your story. These do not need to connect directly to the story, probably should not, but they need to be in the same general context. Same time, same place, same general circumstances.

Then as shown in the other answers use small vignettes in the path of the story to illustrate those traits. ("Use vignettes" would have been a valid answer?) The important part here is that you have to insert the context for those traits within the personality of the character. That means those connections and priorities I was talking about earlier.

Why is this important? Well, back in the "list making" stage I said that otherwise the reader will connect the dots in a random fashion. This is not actually true. They will actually fill in the blanks so they best match a stereotype they are familiar with. Readers will not make new characterizations for you if they can avoid it. So they will match the character to a stereotype unless you supply them the extra data on how the trait illustrated fits the actual character.

This is the exact thing you wanted to avoid and asked about, so the above paragraph, that can be summarized as "attach metadata" is the answer you wanted.

Earlier example in another answer mentioned showing somebody is depressed alcoholic. In such a case you'd insert a small observations such as, "he has been like this since..." or "this is why..." or "but despite being like this he still...". Something to make the trait connected and grounded to the character, not the stereotype.

This also means you can use the low priority traits that do not deserve their own vignette in the vignette for a connected higher priority trait.

In general, you should vignette in the order of priority and so that story relevant traits are familiar to the reader before they become story relevant.

Caveat: This is not intended to work as an an actual "how to" guide. I lack both competence and motivation to write something like that. But it should illustrate the main concept and answer your question?

I would like to provide a contrast to the other answers, don't transfer the knowledge to the reader, except in ways that come natural to the story.

So take for example Ron, a man of a poor family whose daughter is deathly sick. In order to quickly raise the money to save his daughter, he takes up highway robbery.

The MC is traveling down the highway when a man jumps out and yells "throw down your money or your life!". The MC laughs and runs him through with his sword. As far as the reader cares at this point, the highwayman was a bad man and jumped the wrong person, and deserved his fate. The fact his name was Ron, and why he was here is irrelevant to the MC at this point, so the reader doesn't need to know or care.

But if you decide the reader should care, Next chapter the MC runs into the grieving wife and daughter and tracks down the medicine. He gets back when they are having the wake and learns that the highwayman he stabbed was Ron, and the MC now knows the whole story.

Ask yourself "What does the reader need to know?" and "When does the reader need to know it?". Only dive as deep as you need to at any moment, and let the reveals come naturally. If there is something the reader needs to know about a background character that doesn't come up naturally from observation, create an event to show off that characteristic in front of the MC.