I want to start. It's my nature: plunge straight in, worry about the temperature or lack of bathing costume after. So what if some of the characters are a little vapid, indistinct? They'll flesh out as I go along.
They won't. Hence my self-imposed ban on beginning until I know everything about them. And that means everything: physical characteristics, temperament, hopes, fears, secrets, habits. How they vote, how they cry, how they make love. Their childhood, puberty, first kiss. Biggest regret. What would they kill / die for?
Most of this is unlikely to find its way into the prose, but the writer needs to know it. Think of an iceberg, its tip the story, all informed, held up by, the 90% or so hidden below the surface. I need to know how they'd react in any situation. They need life breathing into them to resonate, to be believed. (I actually talk to mine. Out loud.) Give them lines of dialogue to see if they sound authentic. Shout at them, see how they react.
Not that any of this means I retain utter control over them. (One awoke me at 4am today, insisting I change something about her now, lest I forget in the morning.) It's also important to remember people are unpredictable at times; characters must be too. And they should evolve as a result of the things that happen to them, just as we do. So keep them on a leash, yes, but make it a long one.
Name them with care. Try different ones out; they'll let you know when one suits. Don't just use any one now, thinking you'll change it later - they'll become that person and it'll be too late. And watch out for overly artificial monikers. I remember an article by Sven Birkerts for Angi magazine where he described rejecting a story after the opening sentence because the character's name was too contrived, too literary. Harsh, but it happens.
This done, reveal their character slowly, through behaviour and dialogue. Nurture them. Make them the most important people in your life.
Oh, and try not to fall in love with any of them; it'll only end in tears.