I'm currently trying to sketch my son as an exercise. I'm using a photo taken a few years ago. To simplify it, I've reduced the image to black and white and trying out different techniques, papers and materials to see which is easiest to use and get the best results. It's an interesting process. Each time I do a new sketch I end up with something quite different.
Characters are like that, for me anyway. Nailing a character is like trying to achieve a likeness in a sketch. It's all about knowing what to include, and what to leave out. Some details help inform a character, make him real and identifiable. Get something crucial wrong and you throw the whole sketch out of whack. Similarly acting out of character throws the reader out of the story. It's one of those tripping up moments.
So how do you form your characters before you start? Some authorities recommend writing out full bios, or character statements. I think anything helps if it gets you into the mind of that character. Some people start blogs and write it in the character's voice. That's a good one for exploring background and attitudes. But any time spent on this exercise is not wasted, because the Devil, as they say, is in the detail. Get your character right and you'll create someone real that people want to read about. And that's half the battle.