The Americans, but the character I most look forward to seeing on a weekly basis is Martha, the secretary who is an unwilling accomplice to the KGB agent played by Matthew Rhys (another terrific performance). Martha has a small role, and she is played by Alison Wright, who has done very little, according to IMDB. I assume she's a theater actress because there is no way she is this good without having a lot of practice. She does a lot of quirky facial expressions, and comic-style acting, but it feels totally natural and lived-in.
FYI, I have not watched this week's episode, and now that I'm writing this, I'm wondering if Martha survived the week. I'm worried because a) she had a big role in last week's episode and b) the episode ended with her ex-boyfriend following her home. I hope she survives. If not, I hope she gets some more work soon.