The screenplay, by Ring Lardner, Jr., is radically different from the original novel; in the DVD audio commentary, Altman describes the novel as "pretty terrible" and somewhat "racist" (the only major black character has the nickname "Spearchucker"). He claims that the screenplay was used only as a springboard. However, the screenplay itself reveals that, while there is some improvisation in the film, and although Altman moved major sequences around, most sequences are in the screenplay. The main deletion is a subplot of Ho-Jon's return to the 4077th—as a casualty. When Radar steals blood from Henry, it is for Ho-Jon's operation under Trapper and Hawkeye's scalpels. When the surgeons are playing poker after the football game, they are resolutely ignoring a dead body being driven away—Ho-Jon's. The main deviation from the script is the trimming of much of the dialogue.
According to Altman, Lardner was very upset with the liberties taken with his script. Lardner later won Best Academy Award for his screenplay.
The filming process was difficult, because of tensions between the director and his cast. During principal photography, Sutherland and Gould spent a third of their time trying to get Altman fired, although this has been disputed; Altman, relatively new to the filmmaking establishment, at that time lacked the credentials to justify his unorthodox filmmaking process and had a history of turning down work rather than creating a poor-quality product. Altman: "I had practice working for people who don't care about quality, and I learned how to sneak it in." Altman later commented that if he had known about Gould and Sutherland, he would have resigned. Gould later sent a letter of apology, and Altman used him in some of his later works, but he never worked with Sutherland again.
There were only a few uses of loudspeaker announcements in the original cut. When Altman realized he needed more structure to his largely episodic film, editor Danford Greene suggested using more loudspeaker announcements to frame different episodes of the story. Greene took a second-unit crew and filmed additional shots of the speakers. On the same night that these scenes were shot, American astronauts landed on the moon.
During production, a caption that mentions the Korean setting was added to the beginning of the film, at the request of 20th Century Fox studios. The Korean War is explicitly referenced in announcements on the camp public address system and during a radio announcement that plays while Hawkeye and Trapper are putting in Col. Merrill's office which also cites the film as taking place in 1951.
In his director's commentary on the DVD release, Altman says that MASH was the first major studio film to use the word "fuck" in its dialogue. The word is spoken during the football game near the end of the film by "The Painless Pole" when he says to an opposing football player, "All right, Bud, your fucking head is coming right off!" The actor, John Schuck, has said in several interviews that Altman encouraged ad-libbing, and that particular statement made it into the film without a second thought. Interestingly, the offending word was not censored during a late-night broadcast of the film on ABC in 1985; subsequent broadcasts of the film on network television have the word removed altogether.