Peter Guillam is important to Smiley in both Le Carre's Tinker, Tailor, Soldier, Spy and Smiley's People. Peter is young, athletic, and courageous. He is devoted to the sedentary George Smiley and assists him whenever action is required. Peter has a more important role in Tinker, Tailor, Soldier, Spy than in Smiley's People. As a man who is still employed by the Circus, he is able to steal files for Smiley to study as he tries to unearth the mole by reading about the past in his lonely hotel room. Peter has a touch of the James Bond in his character. He loves fast cars. He takes serious risks. He serves Smiley as driver, companion, gofer, confidant, bodyguard, and an insider who can spy upon the spies. He has a smaller role in Smiley's People because John Le Carre evidently decided that his readers enjoyed Smiley for himself and didn't really want the kind of action and excitement that characterized spies like James Bond.
Peter Guillam also serves as an interlocutor with Smiley. Some of the important exposition in both novels is conveyed through the dialogue exchanged between these friends, just as is the case with Sherlock Holmes and Dr. Watson. In Tinker, Tailor, Soldier, Spy, Peter and Smiley stop off at a second-rate roadside Italian restaurant, where Smiley tells his young disciple about the one time he met Karla in person long ago. Karla was in a prison in India waiting to be sent back to Moscow. Smiley had tried to persuade him to defect to the West. Peter listens and learns. This is one of his more important functions in both novels. He is someone for Smiley to talk to in confidence. He is just about the only person in the Circus or in the entire government whom Smiley can trust as long as the mole remains unidentified. In Tinker, Tailor, Soldier, Spy, Peter is head of the Scalp Hunters. Those are the boys who do the dirty work. They are sent all over the world on dangerous missions, and no doubt they commit murders on occasion. In Smiley's People, Peter is attached to the British Embassy in Paris but still working as a secret agent, what Connie Sachs would call a "hood." Peter is married. He is slowing down a little with advancing age, but he can still be useful to Smiley because of his connections with the Circus, his audacious insubordination, and his loyalty to his old friend. He is with Smiley when the mole is unmasked in Tinker, Tailor, Soldier, Spy, and Peter is with him years later in Smiley's People on the night that Karla defects to the West.