The film is about Harvey Milk, California's first openly gay elected official, who was shot and killed, along with the city's mayor, in 1978 by Dan White, another politician who was fighting to get his job back after he resigned his position.
"I had a very visceral reaction to the script," Brolin, who plays Milk's assassin, told reporters in New York Wednesday.
"I read the script and cried and Gus had also sent me the 1984 amazing documentary ('The Times of Harvey Milk,') that I watched with my daughter and both of us were crying at the end of that," Brolin recalled. "So, it was one of those things that was less about the character and more about the story, the fact that we were so moved by it.
"I think the last time I felt like that was -- I did a movie a long time ago called 'Flirting with Disaster' and I remember watching it and I was so happy to be in the film. I was actually watching the film and was able to objectify and go: 'I'm just happy I'm in this film. I love that this film exists.' And it was the same thing with this."
Brolin is best known for his roles in the movies "W," "American Gangster" and "No Country for Old Men."