Noel Streatfeild had a "bus test" for whether she'd developed her characters well and deeply enough: if you saw the family on a bus, would you recognise them? (If you're a Streatfeild fan that's a great site, by the way). It's obvious what the central characters - the ones Forster called round characters - in your novel need, even if it's not so obvious how to set about it. But what about flat characters? What if the world you're working with has a huge number of peripheral characters? How do you bring a bit of life to them, make us believe in them, without without sinking the novel?
It's most obvious at the beginning, when a reader is working out how this novel works. There are lots of ways a novel can work, but it won't work at all if the reader isn't shown how to read it. And we hold on to what we know about this character so far if we think it'll matter later so, roughly, we need our expectations to be fulfilled: the proportion of how much we know should roughly be rewarded by how important they are in the story. Well, never say always: Zadie Smith started White Teeth with a suicide by someone who was fundamentally not important in the novel: what was important was the world that body plummeted through, and what happened next.I do know people for whom that was supremely irritating, and others for whom it was just right for Smith's project in the novel, which is full of the apparent superfluities and redundancies of the modern urban world. You pays your literary money and you takes your choice.
But it's also important to realise that while the amount of detail you spend on a character is one clue for the reader, the amount of depth you give is a different kind of clue, and you need to know what you're doing with each. This kind of thing, I'd suggest, is detail, and could be about someone the narrator's only just met:
I looked at Wendy. She was chewing gum so hard she looked like a cow who's drunk too much coffee.
Whereas here, the narrator has obviously known Wendy, at least slightly, for a while:
I looked at Wendy. Would she still be chewing gum at her own funeral?
And it's the kind of detail which can very economically evoke the characters the school is filled with and the way the narrator's feeling about them. Most readers will take it as just that - a flash of a real person, a whiff of Wrigleys, the kind of thing we experience on the bus of strangers who we'll never know. But even external details need to be in proportion. Subject to what's right for the narrator's voice and point of view, I'd suggest that you wouldn't give us three paragraphs of externals about clothes, shoes, books that Wendy's carrying or stuff in her bag, if she was never appearing again, or only as a walk-on. Readers are hugely experienced in reading the signs of whom to take notice of (a fact that good crime writers twist to their advantage) so I'd suggest you keep an eye on what signals you're sending by the way/amount you describe characters.
Depth is slightly different. You even more certainly wouldn't give us a three paragraphs about a character's privileged childhood and shock of being orphaned, if they're never to appear again, or only have a walk-on in the last chapter. Even this much:
I looked at Wendy. Could what she'd said about her stepmother really be true?
is the kind of thing which alerts the reader to there being more to all this. If you like, it's a little spark of the instability I was talking about in Wake up and re-write. Not only is this a character the narrator knows as a personality in the wider sense - and the implication is that she'll be important to the reader too - but the reader now knows something which is likely to get more important... A little bit of our attention marks it down, as we read on. And if we don't, eventually, get this fleshed out into a chunk of character-in-action - i.e. drama - then we feel uneasy - a sense of loose ends, an irritation that we've been holding on, waiting for it to matter and it hasn't. Whereas this:
I looked at Wendy. Only last week I was telling her off for bullying her little brother.
seems to me somewhere between the two. It says more about Wendy's character than the gum does, but it's an incident which might just be part of life's rich tapestry at Itchtown Comprehensive: it's self-contained. While this:
I looked at Wendy. Only last week I was telling her off for bullying her little brother James.
sets the reader up to expect James also to be someone we need to know about. Did you mean to do that?
And if you do start your novel with waking up, you need to be even more sure that the other body in the bed is also proportionate in both - shall we say - depth, and detail. It's not that they need to turn out to be Chief Love Interest, or Chief Villain. But don't forget that the reader might just spend the rest of the novel waiting to find out which they are. If writing fiction is all about giving the reader what they want, and also some things they didn't know they wanted, it's also about handling their expectations, and turning them into satisfaction.