Just because I believe in fast-drafting a first draft of a novel, does not mean I don't believe in deep characters and plotlines. I do a lot of prep work before fast-drafting to get to know who my characters are at a gut level, like how they feel about their mothers and fathers, what's the worst thing a parent has done or said to them, how they still carry around some of their most embarrassing moments...
However, even with a lot of prep work, I don't like to nail anything down in stone until after my first draft. To me, fast-drafting is as much about exploration as it is about finishing a book. Often I'll get to the end of writing a draft before I really know what the book should be about. And when I go back to read the draft over, I hardly recognize many of the scenes or character traits that have shown up. To me, that is the characters showing me who they are, rather than the other way around.
There are a lot of things that an author needs to think about when crafting a satisfying novel, but I like to take the fast-drafting phase--only 30 days--and spend that time not thinking, and instead just feeling what's happening with my characters and which direction I feel they want to go.
So if we're talking about depth, to be honest, I find more of that as a fast-drafter than I ever did before.