In 1976, in the quirky Western The Missouri Breaks (1976), he plays part of a group of men being hunted by Marlon Brando. That same year, Apocalypse Now (1979), started filming (to be released in 1979), where he plays part of a group of men who are hunting Brando's character.

Has a collection of James Dean memorabilia and more than 200 hats.

In Apocalypse Now (1979), his character ("Chef") is yelling for the Playboy Playmates from the crowd, and one of them is played by Colleen Camp, who, four years later, would be his contented hippie wife in Valley Girl (1983).

Played mystery novelist Dashiell Hammett twice; first as the lead in Hammett (1982), then as a supporting role in Citizen Cohn (1992).

Is part of Francis Ford Coppola's Zoetrope family, having appeared in The Conversation (1974), Apocalypse Now (1979), and One from the Heart (1981), which were directed by Coppola, and The Stone Boy (1984), which Coppola produced.

Acted in two movies with Robert Duvall - Apocalypse Now (1979) and Falling Down (1993); however, their characters do not interact or share scenes in either movie. He also acted with Duvall in the miniseries Lonesome Dove (1989).

Although he had a relatively small role in The Conversation (1974), it is an extremely important one: he and Cindy Williams are the two people having that titular conversation (recorded by Gene Hackman, and their voices are heard throughout the film).

In 1986-87 he spent a lot of time playing law enforcement agents on television: as real-life Detective Bob Keppel in the Ted Bundy two-part television movie, The Deliberate Stranger (1986), and as the fictional Capt. Richard Jenko on 21 Jump Street (1987).

Appeared alongside Harry Dean Stanton in two movies that weren't initially well-received but have since acquired a cult following: The Missouri Breaks (1976), and One from the Heart (1981). They played good buddies in both.

Appeared for the first time at the popular Hollywood Show (Autograph Convention in Los Angeles) in 2018.

As of 2022, he has appeared in two films that were nominated for the Best Picture Oscar: The Conversation (1974) and Apocalypse Now (1979). Both were directed by Francis Ford Coppola.

Acted with Jack Nicholson in Arthur Penn's The Missouri Breaks (1976), and then acted with Nicholson in the Chinatown (1974) remake, The Two Jakes (1990) directed by Nicholson.

In both The Don Is Dead (1973) and The Gravy Train (aka The Dion Brothers (1974)) he played one of a pair of outlaw brothers, with Al Lettieri and Stacy Keach, respectively.

In 1966, Forrest began acting on stage in an off-Broadway production of "Viet Rock", the theatrical version of which marked his film debut.

In Apocalypse Now (1979), his character "Chef" shouts the line, "Never get out of the boat!", which is later recited by Capt. Willard (played by Martin Sheen).