While in the west coast company of "Silhouettes", Forrest made his film debut, co-starring in "When the Legends Die" (1972) for which he won a Golden Globe as most promising newcomer. He went on to make a handful of films in the 70s, including "The Conversation" (1974, his first of four films with Francis Ford Coppola) and "The Missouri Breaks" (1976). His two biggest hits came in 1979, as one of the ill-fated grunts in Coppola's "Apocalypse Now" and as Bette Midler's lover in "The Rose" which netted him a Supporting Actor Oscar nomination.
Forrest has worked steadily in films since, but with uneven success: there were critical kudos for the title role of "Hammett" (1982) in Wim Wenders' moody, cerebral essay in film noir; for his hippie father to the title character in Martha Coolidge's clever "Valley Girl" (1983); as Eddie Dean in Coppola's "Tucker: The Man and His Dream" (1988); and as the relentless prosecuting attorney in Costa-Gavras' thriller "Music Box" (1989). But there have been as many less noteworthy ventures, including Coppola's "One From the Heart" (1982), "Return" (1985), "The Two Jakes" (1990), Joel Schumacher's reviled "Falling Down" (1993), and "Lassie" (1994).
TV has provided a steady paycheck to Forrest, beginning with "Larry", his debut TV-movie for CBS. He has appeared in numerous TV-movies and mini-series. These have included such popular hits as "Ruby and Oswald" (as Lee Harvey Oswald in this 1978 CBS movie), opposite Ann-Margret in "Who Will Love My Children?" (ABC, 1983), as Ted Bundy's bete noir in "The Deliberate Stranger" (NBC, 1986), as Blue Duck in "Lonesome Dove" (CBS, 1989), and reprising his Dashiell Hammett role in "Citizen Cohn" (HBO, 1992). Forrest made one venture into series TV, in the first six episodes of the popular Fox show "21 Jump Street".