Doing It for the Art: Beyond the effort he made to get the movie off the ground in the first place, Peter Sellers gained weight to play Chance despite chronic and worsening heart problems — and he hated how he looked this way — solely because he thought Chance would be overweight given his sedentary lifestyle. He lost the weight as fast as he could after filming, which may or may not have been a contributing factor to his death.
Dyeing For Your Art: Not quite an aversion — while Sellers let his gray hair show, unlike in most of his later films, he did have it cut.
Fake American: Peter Sellers, a Brit, as Chance. In the film, Chance lives in Washington, D.C., but Sellers intentionally plays him without any kind of specific regional dialect/accent as per a description in the book. To pull this off, Sellers took (British) Stan Laurel's voice and gave it an American accent. (Sellers was a huge fan of Laurel and Hardy and openly admitted to using Laurel's screen persona to inform his performance as Chance, which makes this a big Shout Out of sorts.)
Saved from Development Hell: It took a long time getting the rights from Kosinski (he wanted to make a film version on his own; he did co-write the film that was made), as well as getting financial backing.
Solely because of the "I like to watch" scene, Laurence Olivier turned down the role of Ben Rand, the role which Melvyn Douglas won a Supporting Actor Oscar for.
At one point in the mid-'70s, it was announced that Gore Vidal was going to write the screenplay.
The filming script's ending was significantly different than what was ultimately conceived and used. Eve follows Chance after he wanders away and they head back together. He leaves his umbrella behind to shield a small tree — that would have been the highly symbolic final shot. This ending was shot, and a publicity still from the press kit was drawn from it, but the actual scene didn't surface until the 2009 Blu-Ray disc.