I think it's one of my strengths. Granted, one or two characters in Angel Odyssey need a bit more work but on the whole I can usually envision a character and go pretty deep into who they are, where they come from, what their motivations are. I do this pretty quickly actually. Maybe you can credit all my wasted time playing Role Playing Games and being a Game Master. But honestly, it goes back further than that. I used to make up stories about people I'd see or even movie characters. I used to play "Star Wars" in my back yard with the neighbor kids and the plots we came up with had way more drama and action that those bloody prequels.
But fast or slow, you need to know all your characters intimately, even the bad guys. Especially the bad guys. If you don't know your characters, they won't act the way they should. Your warrior/soldier types won't feel or sound real. Neither will your crooks, your women (or any other character of the opposite gender), your cops or your shop keepers. You need to be able to put on your character's clothes and live in them for as long as you need to in order to understand them. There are some great character creation checklist available online or even in some roleplaying games, like the World of Darkness games. Here's one from Holly Lisle's page: http://hollylisle.com/index.php/Workshops/the-character-workshop-designing-a-life.html
Think about your characters, then write them. The pay off in verisimilitude and minor details will pay off, even if most of what you know about the characters doesn't end up on the page.