"Deathwatch" is the 1966 adaptation of the Jean Genet play, starring Leonard Nimoy, Paul Mazursky and Michael Forest (who played Apollo in the TOS episode "Who Mourns for Adonais").
Here's several links. I've taken this quote from the first link:
"The Murderer, The Thief and The Homo: Director Vic Morrow keeps the cipherlike characterizations but brings a cinematic quality to Genet's one-set stage play. Leonard Nimoy (who coproduced the film with Morrow) is the small-time thief who battles with queer Paul Mazursky over illiterate, muscular king convict Michael Forest. . .Deathwatch is a bold gay-themed mainstream movie which vanished rapidly on release and has never been screened since."
Here's one of the movies iconic images:
And here's the New York Times June 28, 1967 review: