I actually write an interview. And this isn't something I write right away.
You can usually find me adding a question or two to the interview from time to time.
Another thing I do is a constant stream of thought about the character's inner thoughts.
But all in questions.
Here's an excerpt I was writing before I came here.
Perhaps he was afraid of facing what life really was about….What was life about?….Wasn’t it what people made of it? Yes it was….he did what he wanted. He could do it for hours without end! But should he? What was wrong with doing things through hours without end? Were there other things he could do? But he didn’t want to do those things. What was stopping him from feeling comfortable doing other things.
Was it other people? Did he really like to socialize?
And so on for a growing number of pages.
Something that helps, and I came up with this just now, might be putting your characters facing a fictional character you already know.
For example, make your character face Dracula. Or Harry Potter or the Whale from Moby Dick. or the evil witch from Snow White.
Yes, OUT OF NOWHERE.
Even if you're just going to write "He wondered why the hell that poor man spent his life sucking blood from other people's throats...? And why he was after him in a crowded street? Why had Dracula met him in his work place?"
I must say that I also have pictures of my characters in my computer. And I have blue prints and bird's-eye view drawings of scenes and such.