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 Racines (1977), the Emmy-winning role that made him a star.
The interior of Gale Sayers' house was the interior set of Darrin and Samantha's house from Ma sorcière bien aimée (1964). This film aired during that show's eighth and final season (one day before The Warlock in the Gray Flannel Suit (1971)).
Michel Legrand's title theme found a life beyond the film that inspired it. At the time, it was very unusual for a theme song from a made-for-television movie to hit the pop charts because a network film would, in theory, be shown only once. However, this film was such a ratings bonanza that the heart-wrenching theme song was heard by millions of viewers the night the film premiered and was much hummed in the weeks and months that followed. There was not enough background music in the film to justify a long-playing soundtrack album, so Legrand recorded "Brian's Song Themes and Variations", which also included selections from a host of his many other film projects. Bell Records, which released the album, concurrently issued a 45rpm single of "Brian's Song", (backed with Legrand's theme from Le messager (1971)), and the single would go on to top the music charts for the remainder of the year. Incredibly, the theme from this film was so iconic that it was incorporated again in Brian's Song (2001).