Benedict Cumberbatch describes his role as Peter Guillam as the ultimate acting exercise: "I've always wanted to play a spy - you are NEVER what you seem."
John le Carré based the character of Karla (played by Michael Sarne) on the KGB's Major General Rem Krassilnikov who was a counter-intelligence spy for the KGB's State Security Committee.
When Smiley is first shown waiting on phone in the secret flat, you can see his shoes removed and under table. In the next scene a few seconds later, he is shown again with shoes on, which he then proceeds to remove.
When Smiley and Esterhase meet on the runway, the plane that stops behind them has the left prop blade to one side. When moments later Peter is shown opening the door, the same prop is now straight up.
Trailer 1 uses the Wolf Suite by Danny Elfman from The Wolfman score. The second trailer uses parts of Frankenstein's Monster from the X-Men: First Class score by Henry Jackman.
Q
What is the song when Peter Guillam attempts to steal the files?
A
'Mr. Wu's A Window Cleaner Now' by George Formby. The complete list of songs from the movie can be found here with scene descriptions.