One of my current favourite screenwriting books is My Story Can Beat Up Your Story by Jeffrey Alan Schechter, largely because of what he says about character. I won't give away all the good stuff for free (buy the book, you'll like it), but he talks about six character types that you can use in addition to hero and villain: protector, deflector, believer, doubter, thinker and feeler. There's a bit more to them than just those qualities, but what I like is that the label doesn't just tell you who they are, it tells you how they'll tend to act or react in any situation. So assigning your supporting characters one of these labels helps to shape their actions, and their actions shape their characters anyway, so they become more rounded and individual...

Never heard of the book before, thanks for the recommendation. Sounds in the ballpark of the approach I promote about the Protagonist's psychological journey and Nemesis, Attractor, Mentor, Trickster. And that is the key to this question -- which by the way is a hugely important one. In a screenplay, every character has a function. And I would argue that function is tied, directly or indirectly, to the Protagonist's metamorphosis. So that is a major filter we use when developing and sorting through our cast of characters.