question about sign

At the end of There Will Be Blood, Daniel Day Lewis's character's son comes to see him and they have a conversation through a sign language interpreter. The son signs sometimes to the interpreter and sometimes to his father, but keeps his face completely impassive throughout.

Now, whenever I've seen someone signing, their facial expressions are a huge part of what they're communicating. Is it possible to sign with your face completely blank? Is much of the meaning lost, or is it just less expressive or something?

I'm also English, and whenever I've seen someone signing it's almost certainly been BSL, so I suppose it's also possible that this is a BSL/ASL difference.

Thank you!