When Jessica Chastaine is working on a part, she says, she makes two lists: "One: everything my character says about herself, and Two: everything everyone else says about her."
It's a good technique, and I'm sure she's not unique in using it: it sounds like a classic Drama School exercise. And, as so often, listening to how actors work is illuminating for writers. If our first draft is like the kind of improvising that goes on when actors and a playwright are devising a piece, and our obsession with finding the right verb is almost indistiguishable from Stanislavski's, then Chastaine's trick isn't even a parallel to what we could do: it's a genuine tool for us in working with characters too.
But what I particularly like about this tool (as opposed to the more usual idea of lists of likes and dislikes, dreams, cars, or, heaven save us, star signs) is that it embodies some fundamentals about fiction. First, fiction consists of characters-in-interaction, and as soon as you start thinking about how one character sees another, you're setting the seeds for what those interactions will be, and what they'll be like. So far, so much like drama.
But fiction - unlike drama - can go inside heads, and get directly at what we tell ourselves, as well as what we tell other people. As the psychotherapists put it, it's the stories we make internally about ourselves and our actions, to understand our own lives past and present, that shape what we do next, and in fiction the writer can actually take the reader to those stories. Thinking about this might help you to shape the voice of your characters, too, since voice grows from the interplay of what each character sees and says, and how they see and say it - and both what and how is a product of those self-told stories.
Lots of actors would say that the most fascinating characters to play are the ones where the gap between what the character says of him or herself, and what the other characters do, is the biggest. Comedy lies in that gap and The Office is entirely built in it: David Brent the exemplar of someone where the gap is vast, and Orsino isn't so different. But tragedy's built in the gap too: Coriolanus is the archetypal self-blind tragic figure, but Lear, in trying and failing to align what his daughters say about him, with how he sees himself, opens up the gap he was trying to close, and falls into it. Only the Fool, whose job it is to speak the truths that others don't know about themselves, sees what's going to happen.
But there's another thing. "What my character says about herself" doesn't - at least in the clip I heard - distinguish what she says to herself, and what she says aloud. But most of us have some degree of gap between the two, if only for the most public occasions: we may make much of failings to beat possible critics to it; we often don't admit to insecurities or fears; or we don't admit to things we're proud of in case we seem conceited or tactless. So that might be another interesting list to make: what does your character believe deep down? What does she admit to, to whom? How big is the gap?
One last thought. I've been going on a lot, recently, about imagining-on-paper as brainstorming, not planning, and Chastaine's comment made me see - yes - a grid. Would it be interesting to draw one up, like those old times-table squares? If the characters down the side are the ones doing "What she says", and the row across the top are the person "Who she's talking about", you've got a square for what Lear thinks of Regan, and a square for what Regan thinks of Lear. Would it be worth pondering about these individual pairs? All of them? Don't worry if the idea makes you come out in a rash, but it would - I'd suggest - be very interesting to fill in.