Salvatore (Sal) Mineo appeared in Escape from the Planet of the Apes playing the role of Doctor Milo. The New York-born actor's highest-profile role had been that of John "Plato" Crawford opposite James Dean in the film Rebel Without A Cause (1955). In addition to his many TV and film appearances, Mineo launched a brief career as a pop singer on the Epic Records label with two charting singles in 1957 followed by a 1958 album titled Sal.
Mineo was offered the role of the chimp scientist in Escape.. at the recommendation of his friend Roddy McDowall, whom he had befriended while filming The Greatest Story Ever Told (1963) on the same Utah location later used for Planet of the Apes. It was reported at the end of November 1970 - days before filming began - that producer Arthur P. Jacobs had added the new ape character in a casting switch to include the two-time Oscar nominee. It has also been suggested that the script was later re-written to kill the character off earlier than planned because Mineo found the makeup so uncomfortable, but an early screenplay dated 2 October 1970 already included Milo and depicted his death as it is seen in the movie. It is true, however, that Mineo hated the role and took it purely for the money. He was embarrassed that an actor of his calibre was unrecognizable beneath monkey makeup, and suffered severe panic attacks brought on by the claustrophobic ape prosthetic appliances. McDowall recalled seeing the terror in his eyes and warned the director he was going to have trouble with Sal. Kim Hunter remembered, "We did have to hug Sal a lot. It was very, very difficult for him being confined in the appliances. He was not comfortable at all being a chimpanzee." Milo's death scene was shot on 22 and 23 December 1970, and Mineo did not attend the movie opening or participate in any promotional publicity. Escape from the Planet of the Apes would be Mineo's last feature film performance before his murder in 1976.