Susan Sarandon, a longtime gay ally, plays a lesbian grappling with her grandchild’s transgender identity in the upcoming Weinstein Co. drama “Three Generations.”

When did you first become an LGBT ally?
If you’re involved in the arts in any way, shape or form, your whole life is completely integrated with gay artists, and creative people who are gay. I started speaking in public in the early days of the AIDS epidemic when there were huge demonstrations that didn’t even find their way into the New York Times. Having lost a lot of very close friends, who were given the message to die in shame somewhere out of sight, that activated me very quickly.

Did you know it would be controversial to play a lesbian in 1983’s “The Hunger”?
I never went in anticipating the reaction it caused. I thought it was an interesting project. Who wouldn’t want to go to bed with Catherine Deneuve? That was a great first experience.

Was that really your first time with a woman?
Yes. I started at the top of the food chain for sure.

Do you think movies have changed people’s opinions about gay marriage?
I don’t know about movies. I think Ellen DeGeneres did. I think there are certainly people who once they were out and married and especially raising children, they have been positive role models. I don’t know if there have been that many films that have been great about gay marriage.

There haven’t been very many gay romances since “Brokeback Mountain.”
That was tortured. I think when it will be extraordinary is when you have stories about life or mysteries or science fiction where you have two people of the same gender who are married but that’s not what the story is about.

Why hasn’t Hollywood gotten there yet?
If it’s hard for studio executives to imagine themselves having a woman in a lead, they probably need to have some fabulous person write a script that envisions a gay couple married as an afterthought. Hollywood is not political. Look at the number of films that are done that challenge the status quo.

Can you tell me about “Three Generations.”
First of all, we’re going to get a new name. if you have any suggestions, we’re open. “Three Generations” is a story about a family comprised of a lesbian couple that’s been together for a long time. I own the brownstone, and I allow my daughter (played by Naomi Watts) to raise her daughter (Elle Fanning) there. The father has not been present, and the movie opens with a discussion of how Elle can get the hormones necessary to transition before the next school year. In order to that, she has to get the signature of her father.

The topic sounds timely.
All along, my character says, “Why can’t she be a normal lesbian?” But she’s not a lesbian–she’s a boy. Then at some point, I say to Elle, “I thought you were too young to know what you wanted. I was just afraid. Now I realize who you are and who I love is not changing. The only thing that’s changing are the details.” I think what’s extraordinary about adding transgender to all the evolutions is that our coloring box has that many more possibilities. We have that many more crayons to color outside the lines. That’s an exciting thing.