(written from a Production point of view)
Michael G. "Mike" Barrier (born 8 July 1933; age 83) had a very short television and movie acting career that spanned the 1960s. He is noteworthy for having played the same character, DeSalle, several times in Star Trek: The Original Series in the episodes "The Squire of Gothos", "This Side of Paradise", and "Catspaw".
Barrier filmed his scenes for "The Squire of Gothos" between Friday 28 October 1966 and Friday 4 November 1966 at Desilu Stage 9 and Stage 10. He filmed his scenes for "This Side of Paradise" between Thursday 5 January 1967 and Monday 9 January 1967 on location at the Golden Oak Ranch, and his scenes for "Catspaw" on Tuesday 2 May 1967 at Stage 9.
He made an uncredited appearance in the 1965 movie The Satan Bug, alongside Lawrence Montaigne and James Doohan, who were also uncredited. That film also featured John Anderson. Barrier and Doohan had earlier appeared in a 1962 episode of Gunsmoke together. Ed Peck also appeared in that episode. Barrier and Paul Fix guest-starred in an episode of Ripcord, a series which starred Paul Comi. Barrier's other credits include The Untouchables (1962, with Paul Baxley), My Favorite Martian (1965, starring Ray Walston and directed by David Alexander), Voyage to the Bottom of the Sea (1965, with Barbara Bouchet), Gomer Pyle, U.S.M.C. (1965, with Tige Andrews), Bonanza (1965, with Louise Sorel and directed by Gerd Oswald), Combat! (1966, with David L. Ross), and Mission: Impossible (1966, with William Smithers, Don Marshall, Eddie Paskey, Vic Tayback, and Jack Donner). Barrier's final credit was the 1969 film Angel in my Pocket with Andy Griffith and Lee Meriwether.
After leaving the acting profession to enter law school, he became a legal officer for the US Coast Guard in Long Beach, California.  As of 2007, he is a substitute teacher in southern Washington.