Gore Vidal was trying to make a low budgeted movie about the life of Caligula, the mad Roman emperor, but the funding just wasn't coming. He finally turned to Bob Guccione, the man behind Penthouse Magazine, for the money. Guccione loved the idea, but had one demand - there needed to be extra sex and violence added to the script. Guccione wanted to create an adults-only version of the 1950s epics, and he wanted to do it big. He hired great actors - Peter O'Toole, Helen Mirren, John Gielgud - and pursued the likes of John Huston to direct.
When Guccione couldn't score one of the greats he turned to Tinto Brass, an Italian filmmaker whose Salon Kitty - about a bugged brothel in Berlin during WWII - convinced the Penthouse mogul he could do sex right.
It wasn't the sex that made Vidal disown the movie (it was originally titled Gore Vidal's Caligula), but rather his inability to work within the filmmaking environment. He called directors 'leeches,' and said their job was simply to make what the screenwriter had written come alive, without interpretation. Brass, who was a headstrong man, wouldn't put up with it (and he hated the script; Brass rewrote it with star Malcolm McDowell) and Guccione was forced to kick Vidal off the set.
Vidal sued Guccione, but dropped the suit when his name was taken off the film. He happily kept the 200k he had been paid to write it.
The film went on to become one of the most notorious movies of all time. It's pretty terrible, and the full hardcore version screeches to a halt for poorly shot extra sex scenes (Guccione himself shot the porn stuff), but it's also very strange and interesting. Even without the porn (there are a number of cuts of the film - some are X rated, some are very, very hard R), the movie is depraved and sick and weird. It's sometimes hilariously cheap, sometimes bizarrely serious.
Decades later Vidal made enough peace with the film to appear in a parody trailer called Gore Vidal's Caligula, shot by Italian artist Francesco Vezzoli. It played at the 51st Venice Biennial.