The actor, who plays boyfriend Scott Smith in the movie Milk, was forced to research the politician's life and learn about the gay rights movement before accepting the role.
"I wasn't aware of him, which is sad I think, especially considering I grew up less than an hour away from San Fransisco," he told Attitude.
"So when I heard that Gus Van Sant was doing this movie about this guy called Harvey Milk, I did some research and learned what the story was. It was surprising I didn't know anything about him before.
"I saw that as one of the more important aspects of the movie that it would really get his story out; they're not going to teach it in schools, you know."
Franco added that doing extensive research had helped him get into the character's role.
"I was just focusing on who he was and he didn't seem like a particularly flamboyant personality. I never thought 'I'm playing a gay character and I had better indicate that.'"
Milk is out now in cinemas nationwide and is released on DVD on June 1.