In my office, I have pictures, photographs and drawings of what I imagine my characters to look like, so that every time I look up, I imprint them in my mind. In this case, looks are everything! (So, of course is internal nature, but that's another story...)
Anyway, someone sent me a link to this fabulous site, called You Didn't Get The Part. It's jarring and fascinating to see what your fave films would look like if say, Sean Penn, instead of Brad Pitt, starred in Fight Club.
Go take a look.