I'm generally quite down on people who see writing as some kind of magical spiritual journey - it's more like digging the Channel Tunnel with a teaspoon a lot of the time. But if there is a magical aspect of writing - a part that I can't wholly rationalise - it's the creation and maintenance of characters. To breathe life into these people and see them wander around the page spouting smart-mouthed dialogue is a pleasure I rarely tire of.
A writer acquaintance of mine imagines all of her characters as looking like real people - the dashing English captain might be based on Ewan McGregor, the heroine perhaps Vivien Leigh. There's a certain appeal to this fantasy casting exercise, but I'd be wary of unconsciously transferring the non-physical traits of those actors into the book's characterisations. For my part, I totally struggle to picture any of my characters in my head. I don't have a particularly visual imagination, so this might be part of it, but I think this vagueness actually helps me, because I never risk over-describing my characters' physical attributes. I mean, who really cares what a book character looks like - it's what they say and how they react that's important. I often say that a lot of the fun of writing for me is just winding up the characters and letting them smash into each other!
Somehow I just know what my characters are like and it's that feeling of living inside their skin that I strive for. Method-writing if you will, although it sounds like multiple personality disorder too! I once read an interview with Sofia Coppola just after she made Lost in Translation where she was asked which of the characters in the film most resembled herself, and she replied that they all represented different facets of her own personality (apologies to anyone who heard me boring Meg Rosoff with that same anecdote at the SCBWI conference).
Examining my own work, this is totally true of the three main characters in Back from the Dead - they all represent who I am and who I want to be (and sometimes who I don't want to be). Griff is jittery and anxious, but at the same time pragmatic and resourceful; James is smart in a bookish way, distrustful of the modern world and reliant on sarcasm to mask his crippling self-doubt; Sarah is playful and subversive - constantly trying to push the envelope and see what happens. You could probably argue that the adult characters aren't as well defined, but then this is a kid's book and I have been guilty in the past of not letting the younger characters take control. After all, children are the future of the world - we're just paying for them to live here!