Hey, It's That Guy!: Draba, the Black Best Friend, is played by Woody Strode. He was better known as Draba than for anything else he did, but before he became an actor he broke the color line in the NFL in 1946.
Reality Subtext: The credited screenwriter, Dalton Trumbo, was blacklisted from Hollywood for refusing to name names during the Red Scare. There's even a line where Crassus proclaims "Lists of the disloyal are being compiled," making this explicit.
Not just the screenwriter. The writer of the original novel was also on the blacklist.
Troubled Production: Fighting between Kirk Douglas and Stanley Kubrick, tensions among the cast, a script being repeatedly revised, the production running extremely over-budget (especially for the final battle, a last-second decision that cost over a million dollars), run-ins with studio censors and Moral Guardians alike... this one touches pretty much everything.