Having played football in high school, James Caan was a more accomplished athlete than Billy Dee Williams. In the film, Gale Sayers is supposed to surpass Brian Piccolo in speed and playing ability, so Caan was forced to slow down considerably so that Williams could believably outplay him.
James Caan initially turned down the role of Brian Piccolo, preferring to focus on his film career, but he liked the script so much that he eventually relented.
Louis Gossett Jr. originally was cast as Gale Sayers. A few days before shooting began, Gossett tore his Achilles' tendon while working out. Studio executives quickly hired Billy Dee Williams as a replacement, leaving Gossett depressed over missing his "shot". Producer David L. Wolper promised Gossett the first great role that came along. Wolper cast Gossett as "Fiddler" in Roots (1977), the Emmy-winning role that made him a star.
While it was not mentioned in the film for reasons of delicacy on prime time TV, Brian Piccolo had a metastasized version of testicular cancer ("testicle" was a censored word). He underwent multiple, extremely painful surgeries that, among other things, included a mastectomy, the removal of a chest wall, and a orchiectomy (removal of the testicle). The surgeon was later described as being too gung-ho because at the time and with the cancer at such an advanced state, there was nothing that could actually be done and the best thing should have been to focus on quality of life, as Brian's chances were virtually zero.
In 1974 a doctor named Larry Einhorn developed a regimen that had a dramatic impact on this form of cancer. For someone in Brian's condition it would have given him a 70% chance of survival. For someone whose testicular cancer had not metastasized, the success rate was 99%. Today that form of cancer, when caught at the right stage, is the most treatable form of cancer.
The interior of Gale Sayers' house was the interior set of Darrin and Samantha's house from Bewitched (1964). This film aired during that show's eighth and final season (one day before The Warlock in the Gray Flannel Suit (1971)).