Generally, I draw a quick sketch of how I picture the MC. An example is the one below that I drew a couple minutes ago. I didn't want to use my original because it's loaded with my MC's information and quirks. I really want you guys to come up with your own/give you ideas on how to round out your MC. I say MC because I rarely sketch out the other main characters. They form themselves in my first draft, and when the time comes for re-writes I strengthen them.
So, there's a basic character sketch of my MC. She's an amateur sleuth named Evie Dawson. I always try to set my MC's apart from the inside out. If you've noticed her hair is darker at the ends due to dye job gone wrong a couple years back. Luckily, its almost grown out. No, I don't mention this in my novel because its one of those quirks who have to get t know your MC better. Hopefully, plenty of traits will stand out in your manuscript that helps the reader form the character in their heads. More than likely, they won't see your MC exactly like you do and that's okay. You want your readers to use their imagination.
After sketching out my MC's appearance, I create various boxes that normally fill up an entire page with traits and goals and background info. Then, I put x's through boxes that practically replicate myself. I don't want to create any Mary Sue's after all. Then, I generally dedicate a page or two to bullets on what may or may not happen to her. Slowly, the main plot will create itself and many of the others can be used as subplots.
There's one thing I never plan out however (as weird as it sounds) and its romance. I never plan out who my MC ends up with because normally he makes himself known in the first draft. And, by using this method, I know the relationship develops on its own and I don't really have to worry about forcing any scenes. If by the end of the novel all that happens is a kiss, so be it. If by the end they've had so many obstacles that they're professing their love to one another and doing a heck of a lot more than kissing, so be it.
Like I've said in previous posts, romance isn't my forte. I only incorporate it when it works and use it as a delicious subplot. Also, be sure to include a few things that may surprise your reader about your character so they aren't some cardboard cut out.
Hmmm. . .I guess what I do next is type up a sample chapter of solely dialogue between the main characters so I can develop their special sayings and stuff like that. When I'm done with my character sketching and light outlining, I stop and take a break before diving into the novel. I let the plot stew to be sure nothing else introduces itself. Normally, stuff does emerge, but its only little sidelines moments that I wind up deleting anyway because they don't further the story.
Here's a great quote that I save to my computer with no recollection of where I got it from:
"As authors, it is our duty to create lovable, enticing characters. And do horrible, evil things to them."
To some extent, I listen to this saying. My characters aren't always super lovable. Even Miss Dawson has a snarky side that slowly breaks down throughout her investigation. BUT, I make her snarky side more funny and less bitchy so that way the reader isn't thinking, "Oh my god, what a bitch!"
So, er, there's my basic character sketching. Hope it helped!