Peter Lawford, June Allyson, and Ann Rutherford were all veterans who had spent the better part of their careers at M-G-M. Allyson and Lawford had co-starred together in two films, Vive l'amour (1947) and Les quatre filles du Dr March (1949), while also appearing separately in Fou de girls (1943). This film served as a reunion of sorts, giving them an opportunity to be in the last film shot on the fabled studio backlot before the land was sold.
The last movie to be shot on M-G-M's fabled backlot, They Only Kill Their Masters (1972) offers parting glimpses of many set pieces that had appeared in hundreds of films from the golden era, particularly the police precinct, which uses the facade of Judge Hardy's courthouse in the Andy Hardy series, and the exterior of Kate Bingham's house, which was for many years the Hardy family's home on fictional Carvel Street.
The exotic sports car driven by Peter Lawford is a 1972 Triumph Stag, Triumph's luxury V-8 muscle car of the '70s.
At the end of James Garner's autobiography, The Garner Files, he comments about the experiences of making every film he's been in. But when he gets to They Only Kill Their Masters, all he says is, "I'd rather not talk about it."
In addition to her big entrance late in the movie, June Allyson appears in significantly-placed, very brief appearances throughout the film. She is in a scarf, walking across the town park during the opening credits, as well as in a cut-away close-up of her eyes when Abel Marsh (James Garner) arrives at the vet clinic and meets Murphy for the first time.