Co-writer and director Michael Cimino convinced Christopher Walken to spit in Robert De Niro's face. When Walken actually did it, De Niro was completely shocked, as evidenced by his reaction. In fact, De Niro was so furious about it, he nearly left the set. Cimino later said of Walken, "He's got balls!"
The slapping in the Russian Roulette sequences was one hundred percent authentic. The actors grew very agitated by the constant slapping, which, naturally, added to the realism of the scenes.
John Cazale was very weak with lung cancer when filming began, and for this reason, all of his scenes were filmed first. Michael Cimino knew from the start that Cazale was dying from cancer, but the studio did not. When they found out, they wanted to replace Cazale. When Meryl Streep learned of their intentions, she threatened to quit if they did. Cazale died in March 1978 shortly after filming was completed. He never saw the completed film.
The woman who was given the task of casting the extras in Thailand had much difficulty finding a local to play the vicious-looking individual who runs the game. The first actor hired turned out to be incapable of slapping Robert De Niro in the face. The caster thankfully knew a local Thai man with a particular dislike of Americans, and cast him accordingly. De Niro suggested that Christopher Walken be slapped for real by one of the guards without any warning. The reaction on Walken's face was genuine.
The studio wanted to replace John Cazale when he was ruled un-insurable. Robert De Niro put up the money for the insurance. Cazale died shortly after filming was completed.