Yeah, I read somewhere that for TV, since a lot of the shots are close-ups or torso and above, you have to keep yourself still and not use gestures. Thus, you have to over-act with facial expressions to convey emotion (but of course, not so much that you look fake). DT's elastic face is perfect for that; another fantastic expression actor is Nathan Fillion. And then there's stage, where it's all voice and body acting and very little face acting, since you cannot rely on the audience being able to make out your face from the nosebleed seats.

I had no idea you had mostly never seen Clara. I could rant for pages about her. Basically, she started as Eleven's Impossible Girl, and since the whole story line was about Eleven trying to figure out why she existed at all, Moffat didn't bother to give her a personality or history beyond "chipper". Her attitudes and skill sets changed depending on what the episode needed the companion to do. Then with Twelve, Moffat decided that her whole point was that she's as powerful and wise as the Doctor and that the tension between them is that each of them is trying to mold the other into the image they want. A lot of people enjoyed them, but to me, it was painful to watch.