Many Wikipedia articles about the film adaptations of novels include sections detailing how loyal the movies were to their source material. The Wikipedia entry for Tinker Tailor Soldier Spy does not get into this, so I will.
On the whole, Tomas Alfredson's 2011 film was remarkably loyal to John le Carré's seventh novel. I've said before that it was seemingly unapologetically made for people who've read the book. Not only is the crux of le Carré's main plot preserved, but several portions of dialogue ("the unpaid Bill" and "you're losing your sense of proportion, Connie" are just two examples) are lifted straight from the novel. There are even several easter egg-type nods to the novel in some of the dialogue ("the Dolphin will kill me if I don't").
The changes that were made seem to have been made to help the story flow better as a film. This involved the removal of some minor characters and some event resequencing.
None of the details listed below should spoil anything for people unfamiliar with the novel or film.
- In the novel, le Carré provides information about the backgrounds of the Circus employees suspected of being the mole and provides more character development for Irina. This is not done in the film, presumably for time reasons.
- The Circus bodyguard Fawn does not appear in the film, nor does the Soviet spy Ivlov. The secondary character Camilla, who in the novel is Peter Guillam's lover, is also not in the film.
- The characters of Sam Collins and Jerry Westerby have been merged. In the film, a character named Jerry Westerby assumes Collins's role while there is no character named Sam Collins. The role of Jerry Westerby as it appears in the novel is omitted. This essentially guarantees that, while there have been rumours of a sequel, it will not be The Honourable Schoolboy. That would make Smiley's People the only possible sequel.
- The scene with the Circus Christmas party is unique to the film. The main information the viewer is supposed to take away from that scene is described elsewhere in the novel.
- The order of events surrounding Ricki Tarr's meeting with George Smiley is different.
- The Prideaux mission takes place in Czechoslovakia in the novel and in Hungary in the film. (This is because it was more economical for the filmmakers to shoot in Hungary.)
- The identity of a person responsible for an event towards the very end of the story is hinted at in the novel and made explicit in the film.
- In the novel, it's said that Smiley recruited Toby Esterhase in Vienna, but the film attributes this to Control. The subject matter and result of Smiley's main conversation with Esterhase is also the same in both the novel and film, but he goes about the conversation in a different way.
- Guillam tests some recording equipment in both the novel and the film; in the novel, he sings "Old Man River" but he recites a poem in the film. (I would have paid to hear Benedict Cumberbatch sing "Old Man River," but alas.)
So what's better?
Having read the book and seen the film several times each, I believe the latter is a remarkably good adaptation of the former and that the changes made were reasonable. A completely faithful adaptation of the novel would have been much longer than the completed film, and more like the British miniseries starring Alec Guinness. But that had already been done, so why do it again?
John le Carré was an executive producer on the film, and he consulted during the screenwriting. (He also has a cameo in the Christmas party sequence.) Co-screenwriter Peter Straughan outlined the process of adapting the novel as a film* in a column on the Huffington Post. Straughan writes that le Carré encouraged him and his (late) wife, Bridget O'Connor, to make changes and move things around. He also says le Carré's initial reaction to the film was that he was "chuffed to fuck." If the man himself liked it, who are we to quibble?
* Straughan's blog post is more spoilery than this writeup, just so you know.