Gary Burghoff was the only major-character performer to appear in both the 1970 M*A*S*H film and the television series, playing Radar O'Reilly.
In the film, Burghoff played Corporal Walter “Radar” O'Reilly, the 4077th's remarkably intuitive company clerk. When the television adaptation premiered in 1972, nearly the entire ensemble was recast: Alan Alda replaced Donald Sutherland as Hawkeye, Wayne Rogers replaced Elliott Gould as Trapper John, and Larry Linville replaced Robert Duvall as Frank Burns.
Burghoff's Radar became one of the television show's defining characters. The series expanded him from a brief film role into a naïve but highly capable clerk known for anticipating officers' needs. The television version also gave him the full name Walter Eugene O'Reilly, which was not established in the original film.
Several film actors later made guest appearances on television, but Burghoff was the only regular performer to continue in the same major role. He left the series after its seventh season, returning for the two-part farewell “Goodbye, Radar.”