If you're considering seeing this film, take into consideration the following two points:
1. This film does not have a true expository sequence that explains everything that you've just seen.
2. The majority of anything that can be termed as true "action" occurs within the first ten minutes of the film.
If either of these points comes as a disappointment to you, then Tinker, Tailor, Soldier, Spy is not the film for you. If you're a fan of Mission Impossible and/or Sherlock Holmes and think that this film will be like one of those series, you will be disappointed. You see, John le Carré, the novelist behind the film did work in intelligence and his novels have a much more realist bent to them - and as it happens, spies simply aren't very good if they're drawing attention to themselves by having public shootouts or blowing up restaurants. As if to underline this point, the action in the film begins because of a public incident that forces the head of the "Circus" (think MI:6) into retirement as the scandal blows over.
The bulk of the film is spent trying to discern the identity of a mole at the highest levels of the government. There are conversations and contemplations and terse scenes where you're waiting for someone to get caught by they never do. This movie instead forces you to pay attention, to constantly tease out the relationship of incident A to incident B, where it all fits in in the timeline, and whether or not it's giving you a hint to the identity of the culprit. It's a quiet, tense film with many stretches of true silence - no dialogue, no music, just silence as the characters work things through.