K.L. Smith
Kenneth Lavelle "K.L." Smith  was the actor who played the Klingon captain in the  third season episode . He filmed his scenes on Friday  at Desilu Stage 9.

Smith made guest appearances on numerous television series, including Gunsmoke, Rawhide, The Untouchables, Wagon Train, Bonanza, The Virginian, and Gene Roddenberry's The Lieutenant (starring Gary Lockwood). 

He and fellow Star Trek actors Leonard Nimoy, Lawrence Dobkin, and William Schallert had uncredited appearances in the classic science fiction film Them!. Smith's other feature film credits include The Wild One (1953), Pushover (1954), Oh… Rosalinda! (1955, with Jill Ireland), and the Elvis Presley movies Jailhouse Rock (1957, with Dick Dial and Noble Chissell) and Roustabout (1964, with Lance LeGault, Marianna Hill, Terri Garr, and Carey Foster).

Smith retired from acting in 1971. His last appearance was in the 1972 pilot movie for the short-lived series The Delphi Bureau, starring  actor Laurence Luckinbill.

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Performers
TOS performers