We talked about this in one of my writing classes once, and my professor said it all boiled down to getting a character "in."
"In" what, exactly? Well, jammed in the brain of the reader, of course. And no matter how many people you throw into the equation, that character never gets lost, because you have established them in a strong way, even if they are a minor character.
The question is...how does this seemingly miraculous event occur?
I picked through some stories that had particularly memorable characters for me. Let's have a look.
And on the way from school, walking up the road with her arms full of books, one of the boys had said something about her "dumb baby brother." At this she'd thrown the books on the side of the road and tackled him with every ounce of strength she had, and arrived home with her blouse torn and a big bruise under one eye.
(A Wrinkle In Time, Madeleine L'Engle)
Now, Meg is the main character of the book, so it would be hard for her not to get "in." But I think these lines are where it happened for me, because they suggest half a dozen things at once: that Meg is protective of her brother, that she's a little impulsive, that she's not particularly concerned with her appearance, that she's probably some kind of book nerd. All of those things prove to be true--and crucial, actually-- throughout the course of the story. And this is one of my first tastes, my first and indelible impression of who this character is.
[Mrs. Turpin] stood looming at the head of the magazine table set in the center of it, a living demonstration that the room was inadequate and ridiculous. Her little bright black eyes took in all the patients as she sized up the seating situation.
("Revelation", Flannery O'Connor)
These are the second and third lines of this short story. If you've read it, you know that Mrs. Turpin spends the rest of it judging and assessing all the people and things around her, and it leads to some pretty interesting (and disturbing) consequences. She is, in a word, self-important-- and no one had to tell me so. Just standing at the magazine rack and surveying the room is enough to communicate that to me.
He could make me feel as though he had poked me with a stick, just by looking at me.
(Gilead, Marilynne Robinson)
This is a description of the narrator's one-eyed grandfather, and upon reading it, I somehow feel like I know exactly the sort of man he's talking about, and exactly the sort of look.
These examples are all different, and some probably work better for different readers than the others, but what they have in common is detail and specificity. If the grandfather's stare in the last one was described as simply "piercing," his description probably wouldn't have been as memorable to me, just as saying "Meg was overprotective of Charles Wallace-- she'd once beaten someone up just because they said he was dumb" would not have done much for me. It's Meg's torn blouse and black eye and thrown books that gets her "in" my head.
Sometimes a particularly unique or compelling physical description does it (example 3), sometimes it's a short event (example 1), and sometimes it's a gesture or a stance (example 2), but we need these details to really know a character. And, I think, it's not just that there are details, but unique ones. Not a grandfather with a gray beard, or a young girl with brown hair and glasses, or a large, bossy woman-- there are a lot of those in the world. What works best for me, in these examples, is that each writer picked a distinguishing feature of each character and zoomed in on it early in the story (or in the character's introduction). And that got the characters "in".
So the next time a critique partner or your editor or whomever mixes up two characters while talking about them, or tells you they don't remember who so-and-so is, think about how much you can do with a carefully chosen detail.
Speaking of which....I have some work to do now!