(written from a Production point of view)
Tom Steele (12 June 1909 – 30 October 1990; age 81) was a stuntman and stunt actor who played a Klingon soldier in the Star Trek: The Original Series first season episode "Errand of Mercy", and later a slave in the Star Trek: The Original Series second season episode "Bread and Circuses". He was uncredited for his appearance. He filmed his scenes on Tuesday 12 September 1967 and Wednesday 13 September 1967 on location at Bronson Canyon.
Steele's name is featured on a cast list for "The Alternative Factor" for Monday 21 November 1966, however, he cannot be seen in the finished episode. It might be that his appearance has been cut, or that his name appeared on the list for a different reason. 
Steele was born as Thomas Skeoch in Scotland, United Kingdom and entered the stunt industry in the early '30s. He has doubled actors such as Allan Lane, Jack Randall, George Reeves, George Montgomery, and James Arness and performed stunts and stunt acting roles in over three hundred movies and television series.
Among his extensive resume are films such as Flash Gordon (1936), Forbidden Valley (1938), Buck Rogers (1939), The Oregon Trail (1939), The Masked Marvel (1943), Captain America (1944), Zorro's Black Whip (1944), The Phantom Rider (1946), Son of Zorro (1947), The Thing from Another World (1951, with Kenneth Tobey), South Pacific Trail (1952), Spartacus (1960, with Jean Simmons), Bullit (1961), Diamonds are Forever (1971), Earthquake (1974), The Blues Brothers (1980), and Tough Guys (1986). He worked as part of the stunt crew on the 1954 western Cattle Queen of Montana starring Anthony Caruso and Ronald Reagan, who later went on to be elected the 40th President of the United States.
He worked also on television series such as Soldiers of Fortune, Laramie, Annie Oakley, Zane Grey Theater, Have Gun – Will Travel, The Andy Griffith Show, and Hondo.
Steele passed away on 30 October 1990 in Los Angeles, California. He was 81 years old.