Leonardo DiCaprio's Once Upon A Time in Hollywood actor Rick Dalton swings between moods when he's forced to be himself and not a fictional Western character. There's a good reason for that. DiCaprio and director Quentin Tarantino revealed in an interview that Dalton is suffering from undiagnosed bipolar disorder. 

In a chat with Vanity Fair breaking down the character, Tarantino revealed that Dalton was partially inspired by Pete Duel, a Western television actor who died by suicide in 1971. 

Tarantino said that the death was his first interaction with the idea of suicide and that he was fascinated by it, looking into it and theorizing that the late actor was suffering from an undiagnosed mental condition. When Tarantino and DiCaprio got together to flesh out the character of Dalton in Hollywood, they built up from that idea. 

“We already had Rick [as] a drinker, but the whole thing of undiagnosed bipolar and not knowing how that works, and the weird pendulum swings of emotion that you would have, especially if you don’t have a medical understanding of why you feel that way, that became a really interesting thing that we thought Rick could deal through,” Tarantino said. "That gave Leo a good solid grounding in which to work and to build a character."

The director noted that "we never say the words that he's bipolar" but DiCaprio thinks it comes through as the film follows Dalton around in his day-to-day life rather than saying it overtly in the film.