It's often a tiny little touch that impresses me about an actor's performance on screen. For instance, in one episode of Dexter, where he believes he has been caught, Dexter is walking into someone's office under escort. He has not been arrested, but actor Michael C. Hall holds his hands clasped in front of him, just the way he would if he were handcuffed. That little detail stood out to me as a way of expressing that Dexter feels like he's been caught, even though he doesn't know his status right now. Then there's Carrie-Ann Moss playing a spy on Chuck who has a crush on one of Chuck's coworkers/handlers, played by Adam Baldwin. As she walks away from him, she looks over her shoulder to say "Bye, Casey", and her ankle rolls just a little, putting a wobble in one step. That little motion brings to mind all those awkward crushes of our teenage years into one subtle motion, and I really appreciated its expressiveness. Not a bad impact for two tiny physical motions, is it?
Mokalus of Borg
PS - I'm not sure if either of those things were in the script.
PPS - If so, then they were performed perfectly.