If Sherlock is a study in how a versatile actor like Benedict Cumberbatch is able to convey such a wide range of emotions in portraying such a theatrical character, because Sherlock is definitely a very theatrical character, Cumberbatch's role in Tinker Tailor Soldier Spy as George Smiley's loyal right-hand man Peter Guillam is a study in how such an immensely talented actor like Cumberbatch is equally able to convey so much emotion and make us feel for him and fear for him with so much less. That's the mark of a genius actor right there.
Here is what Gary Oldman, his co-star in Tinker Tailor said about him during the DVD commentary:
"A marvelous young actor."
Even Meryl Streep, the greatest actress of her generation, enjoyed Cumberbatch's acting in Tinker Tailor.
According to Cumberbatch, who met Streep at the Golden Globe Awards. "‘Oh, yes!’ she said. ‘I saw you in Tinker Tailor Soldier Spy. I loved it!’ She meant it. She usually, very politely, manages to wriggle free from people she doesn’t care for. "
A journalist about sums up his acting in the movie:
"The most unheralded of the supporting “Tinker” crew, Cumberbatch, is also the most mesmerizingly good. The “Atonement” star doesn’t have one standout scene in “Tinker”, he has at least three. These are powerful moments where Cumberbatch makes you feel the insane stress his character, Peter Guillam, feels assisting Gary Oldman’s George Smiley in his clandestine investigation and where he has to throw away his personal life for the greater good of his country. It’s fantastic work that will not be forgotten by the acting branch. (…) Simply, Cumberbatch is one of the few contenders this (Oscars) season that if he doesn’t get in it’s a travesty."
-Gregory Elwood, journalist
If you haven't already, you should check out this deleted scene between Peter Guillam and Ciaran Hinds's character Roy Bland, which takes place right after Guillam is asked by Smiley to start spying on the Circus. In this scene, Bland is subtly threatening Guillam, and you can see how Cumberbatch's hand trembles ever so slightly as he is smoking, how already he is beginning to fray at the edges under all the guilt, stress and fear of having to spy on his own people. Superb interplay between these two actors in this scene. Pity that it was excised from the final cut of the movie.