I do know roughly what my characters look like but the thing I pin down first is their main character traits. Eileen, for example, is as awkward as they come but honourable and brave with it. Those traits can lead into interesting plot developments. Eileen would rush in where angels fear to tread.
That kind of thought then makes me think well has she done this and if I decide yes she has to back up my initial thought, I can get some sort of back story for Eileen. I may or may not use some of this back story to flesh out her character in whatever piece I'm working on but I find in knowing the information myself I write with more confidence. I think it wil show in the writing. Certainly when I am reading other works, if the characterization doesn't ring quite true for whatever reason, it stands out and spoils the story for me.
A determined character like Eileen is huge fun to write for and about but there has to be a balance. There has to be someone who can rival her or hold her back. Why? A character like Eileen cries out for something/someone to fight against (whether those fights are literal or not is another thing). Also a character like her would rub plenty of people up the wrong way. My work then takes on a more realistic feel if something of that ends up in my stories. There is no way someone like Eileen could ever have a quiet life.
I also look at what would make my characters vulnerable as this helps me show their good/bad/able to be corrupted or not side. That in turn could throw up ideas for what I could use to threaten those characters and how that could tie in to my overall plot.