The Spy Who Came in from the Cold

The West Berlin office of the Circus, under administrator Alec Leamas (Richard Burton), has not been doing well. He is recalled to London shortly after the death of one of his operatives. Leamas is seemingly demoted to the banking section of the agency. In reality, a carefully staged transformation of Leamas has been arranged by Control (Cyril Cusack), the agency's chief. Now depressed and disgruntled, alcoholic and low on funds, he is quickly spotted by the East German Intelligence Service as a potential defector.
Leamas accepts overtures from German communists to reveal British secrets for payment, and he is interviewed by a man named Peters (Sam Wanamaker) at a coastal house in the Netherlands about what he knows. When the process is later moved to a country villa in East Germany, the interviews become less cordial. It appears Leamas has information that will implicate a powerful East German intelligence officer named Mundt (Peter van Eyck) as a paid informant of the British, but the information is spotty and it frustrates his interrogator, Fiedler (Oskar Werner). When Mundt arrives at the compound and discovers the investigation, he has both Leamas and Fiedler arrested. Mundt himself is eventually arrested.
An East German tribunal ensues to determine the guilt of Mundt, with Leamas appearing as a star witness. Mundt's attorney (George Voskovec) uncovers several discrepancies in Leamas' transformation into an informant. Leamas' credibility collapses when his English girlfriend, an unassuming and idealistic communist named Nan Perry (Claire Bloom) unwittingly reveals that she has received payments from British intelligence. As Leamas' charade unravels and he is forced to admit he is still working as a British agent, Fiedler is escorted from the room as a complicit dupe and Mundt's reputation is vindicated.
Leamas initially believes he has failed in his mission and he will soon be executed. But when Mundt releases him from his cell with an escape plan in tow, he learns that his mission has actually succeeded; Fiedler was the agent to be undermined and Mundt was indeed a British agent. Although this comes as a surprise to Leamas (for he has steadfastly insisted to Fiedler that the Circus could not possibly have run an agent in East Germany without his, Leamas's, knowing about it), he isn't completely shocked by the revelation. As he and Perry sit in a car waiting to be escorted from East Germany, she berates him as being involved in murder: the execution of Fiedler who was guilty of nothing. Leamas, agitated by Perry's naivetÃ©, tells her that her worldview is childish and people are murdered every dayâon both sidesâwhile she lives an insulated life: "What do you think spies are?" he asks. "They are a bunch of seedy squalid bastards like me, little drunkards, queers, henpecked husbands ..."
Leamas and Perry are soon ushered to the Berlin Wall and apparently permitted to leave. But Perry is shot down, apparently by an East German double agent, as she tries to cross the wall. Leamas then looks down from the top of the wall at Perry, while agents from both sides urge him to return to the west. Instead he climbs back down the East German side of the wall and goes to Perry's lifeless body, but is then himself shot dead.

Question: Who appears as a star witness to determine the guilt of Mundt?
Answer: Leamas