Eddie Marsan
I finally saw this film and was very impressed with the cast. I can see why Imelda Staunton got the Oscar nomination. However, the performance that really stood out for me was Eddie Marsan's. The character of Reg could easily have disappeared into the wallpaper, but in basically a handful of lines Marsan transforms him into one of the most insightful and sympathetic people in the film.
I'm always impressed when an actor can take a character that could easily be a simple archetype (in this case a somewhat dull, working-class schlub) and break out beyond those constraints.
I was surprised to discover that I had already seen Eddie in several other excellent support roles in movies like 21 Grams and V for Vendetta.