Jay Roach claimed that many of the scenes featuring Trumbo writing alone at his desk or in the bathtub were improvised by Bryan Cranston while the cameras rolled, and that Cranston was genuinely composing complete sentences on the page.
Steve Martin has written that very early in his career his girlfriend and her "different" family introduced him to new ideas and intellectual opportunities. Her father was Dalton Trumbo. Martin had never heard of Dalton Trumbo.
Dean O'Gorman wrote to Kirk Douglas for his advice about playing him in the film. Douglas wrote back, telling the 38-year-old actor to be professional and trust his instinct: "Playing Kirk Douglas, forget him ... just play the part and you will be fine."
Alan Tudyk entertained his on-screen son, Elijah Miskowski, and the other kids on set by performing the voices of King Candy (Wreck-It Ralph (2012)), Duke (Frozen (2013)), and Alister Krei (Big Hero 6 (2014)). The boys were impressed to discover he was also the marine voice from Halo 3 (2007) and it made Alan the "kid magnet" during the early shooting of Trumbo.
This is the third feature film in four years in which John Goodman played the owner or an important employee of a movie company, following The Artist (2011) and Argo (2012). Interestingly, both prior films won Oscars for Best Picture. Goodman also played a 1960s-era producer of cheap horror movies in Matinee (1993).
In the middle of the movie, Hedda Hopper (Helen Mirren) is asked how her son is doing. She replies that he is doing well and is in the Navy. Hedda Hopper's son was William Hopper, of Perry Mason (1957) fame, who indeed served in the Pacific in the U.S. Navy during WW II.
Dalton Trumbo won two "Best Writing, Motion Picture Story" Academy Awards during the 1950s but was unable to accept either of them, since both movies' credits had used "fronts" (real people who agreed to take credit for the scripts while Trumbo was blacklisted). The first movie for which Trumbo won an Oscar was the Audrey Hepburn-Gregory Peck romantic comedy Roman Holiday (1953). For this movie, Trumbo's front was Ian McLellan Hunter (who actually was also a screenwriter in his own right); Hunter was also later blacklisted. In 1993, after both Trumbo and Hunter were both dead, the Academy attempted to retrieve the Oscar that had been presented to Hunter and present it instead to Trumbo's widow, but Hunter's son, Tim, himself a director (River's Edge (1986), Tex (1982), etc.) refused to relinquish it, so the Academy instead presented Mrs. Trumbo with a new statuette. On Roman Holiday's 2003 DVD release, Trumbo was credited in place of Hunter. The second movie for which Trumbo won an Oscar was the family drama The Brave One (1956). For this film, Trumbo's front was named Robert Rich; unlike Ian McLellan Hunter, Rich was not actually a screenwriter himself but just a nephew of the movie's producers. The Academy re-presented that Oscar statuette to Trumbo in May 1975, roughly a year and a half before Trumbo's death.