I think it's easier for me in fanfic, because I have a mental catalog of the quirks and mannerisms of the actor/character to draw upon. It's the original novel where I'm really getting stuck, because I need a similar catalog for each of my characters and I'm having to build it. Even in those cases where I know my characters' body language really well, it's hard not to be repetitive (for example, I have one character who is very understated, subtle and quiet in all that she does, but constantly referring to her "little smiles" and "tiny nods" is probably going to be quite wearing for the reader, I think).

Looking back over dialogue scenes I've written, though, there's way less uninterrupted dialogue than I actually thought I was writing at the time. I think I can probably get away with knocking off some of the descriptors and just going for the dialogue in some of my iffy cases.