(written from a Production point of view)
He has made guest appearances in many other television series, including Gunsmoke, Have Gun - Will Travel, The Twilight Zone, My Favorite Martian (starring Ray Walston), Mission: Impossible, The Andy Griffith Show, Bonanza, Barnaby Jones (with Lee Meriwether), Quincy (with Garry Walberg and Robert Ito), Starsky & Hutch (starring David Soul), Alice (with Vic Tayback), and Picket Fences (with Ray Walston and Roy Brocksmith), as well as the pilot movie for The Incredible Hulk. He also appeared in the 1972 TV movie Columbo: The Most Crucial Game, along with James Gregory, Susan Howard, and Dean Stockwell. His other TV movies include Who Is the Black Dahlia (1975, with Ronny Cox, John Fiedler, and Sid Haig), Lannigan's Rabbi (1976, with Andrew Robinson), The Girl in the Empty Grave (1977, with Jonathan Banks and James Cromwell), The Immigrants (1978, with Stephen Macht), Samurai (1979, with Michael Pataki), The Scarlett O'Hara War (1980, with Clive Revill and Warren Munson), Amos (1985, with Ray Walston and James Sloyan), and Lucy & Desi: Before the Laughter (1991, with Alan Oppenheimer).
Keefer also has several films to his credit. He had a small role in the highly-acclaimed, Academy Award-winning 1969 western Butch Cassidy and the Sundance Kid, which also featured fellow TOS guest actors Jeff Corey and Ted Cassidy and Star Trek: Deep Space Nine guest star Kenneth Mars. Prior to this, he and another TOS guest star, Whit Bissell, had uncredited roles in The Caine Mutiny, which was released in 1954 and also earned several Academy Award nominations. That same year, Keefer and Bissell co-starred with William Schallert in Riot in Cell Block 11. Keefer and Scallert would go on to appear in 1955's An Annapolis Story and 1958's Torpedo Run (with Richard Carlyle).
In 1966, Keefer appeared in the war comedy The Russians Are Coming, the Russians Are Coming. Also starring in this film were Brian Keith, Andrea Dromm, and Theodore Bikel. (Michael J. Pollard made an uncredited appearance.) In 1970, he co-starred with Gary Lockwood, Paul Winfield, and Graham Jarvis in R.P.M., and the following year, he and his "Assignment: Earth" co-star Robert Lansing appeared in The Grissom Gang, which starred Kim Darby and also featured Hal Baylor. In 1973, he and Kenneth Tobey appeared in the popular film, Walking Tall. That same year, he co-starred with Woody Allen in the latter's acclaimed science fiction comedy Sleeper.
Other film credits include Incident in an Alley (1962, with Clegg Hoyt), The Young Nurses (1973, with Dick Miller), Ace Eli and Rodger of the Skies (1973, with Bill Quinn), Candy Stripe Nurses (1974, with Dick Miller and Bill Erwin), Fire Sale (1977, with Richard Libertini), The Car (1977, with John Rubinstein and Ronny Cox), Creepshow (1982, in the same segment which features Adrienne Barbeau, Fritz Weaver, Robert Harper, the film also starred Jon Lormer), and The Marrying Man (1991, with Jeremy Roberts. His latest film appearance was in the 1997 comedy Liar, Liar, which also featured Anne Haney and Randy Oglesby.
- Samurai (1979; with Michael Pataki, Walt Davis, Johnny Haymer, Bob Minor, Tom Lupo and Gregory J. Barnett)