Straughan: We knew from the beginning that we had to move it deeper into the story [in the movie] because we had to introduce George Smiley. Didn’t you say, Gary, that when you were shooting this scene that you maintained a Smiley silence which was freaking Tom out?
Oldman: Yes, it discombobulated him.
Straughan: You stayed in character.
Oldman: You’ve got a great excuse when you are playing Smiley. I would get to the set, and I was ready and I would just sit and wait. There is no particular method of how you approach a role but you find one. It was a nice thing for me to get there and be ready. There was a set up in this scene and we had about 40 minutes of downtime. I stayed sitting in the chair. And Tom was just talking and talking and talking and I was just sitting there like a statue, without realizing it. Tomas was watching this and he said, “Do you know that you have not spoken for 45 minutes?” I just watched Tom burble on. That is one of the secrets of George and how he gets people to open up.
From this interview.