In real life, 'James Caan' (who had been a football player in high school) was a much better athlete than Billy Dee Williams; but in the movie Williams' character, Gale Sayers, was supposed to surpass Cann's character, Brian Piccolo, in playing ability. So when they were racing on film, Caan was forced to slow down considerably so that Williams could beat him.
Louis Gossett Jr. was originally cast as Gale Sayers. Just days before shooting began, Gossett tore his Achilles' tendon while working out for the film. The studio execs scrambled, and quickly hired Billy Dee Williams as a replacement. Gossett, depressed over missing his "shot," was promised by producer David L. Wolper the first great role that came along. About six years later, Wolper called Gossett to play "Fiddler" in Roots (1977), the Emmy-winning role that made him a star.
It is interesting to note that a year after Brian's Song was released, both James Caan and Billy Dee Williams were in two of the best 1972 movies that were Oscar nominated for best picture in 1972: James Caan - 'The Godfather' (Caan was also nominated as best supporting actor) and Billy Dee Williams played the male lead in 'Lady Sings the Blues'.