That's a good question. 3D animation could help capture the non-humans, but more and more movies are being made that really blur the line between the two forms.

The first hard and fast rule I would have is that Steff would have to be played by somebody biologically male... none of this casting a woman as the "tranny" and giving her bad cheek make-up. If it were up to me, I think I'd prefer to do an open casting call to find unknowns who really suit the characters, because nobody in Hollywood really matches my mental images of them.