Rather than cast an actor to portray Joseph McCarthy, the film uses actual footage of the senator. It has been claimed, widely but without evidence, that test audiences, unaware that only archival footage was used in McCarthy's depiction, felt that the "performer" who "played" him was overacting. IMDb, for instance, states without attribution, "Clooney had said that when the movie had undergone test screenings, audience members felt that the McCarthy character was overacting a bit, not realizing that it was the actual McCarthy through archive footage." In fact, there are no authoritative reports of any particular such test audience reaction, and the story remains a popular but completely unsubstantiated rumor.
George Clooney, while discussing the film as part of the Newseum's "Reel Journalism" series on Jan. 26, 2009, confirmed this rumor, to a degree. The filmmakers themselves decided not to cast anyone as McCarthy because anyone "playing McCarthy" would seem like they were overacting, even if they were simply mirroring McCarthy's mannerisms. The role was never cast to begin with as a result. Interestingly, Clooney also said this was the prime reason the film was in black and white -- to match the black and white footage of McCarthy.