BEING YOUNG and thin generally doesn't cause problems for actresses in Hollywood, but both were getting between Zoe Saldana and the role she desperately wanted in "Infinitely Polar Bear."

The director, Maya Forbes, "wanted me to gain weight. I guess she wanted me to look more like a mother," said Saldana, laughing at the irony of it all. Late last year, Saldana gave birth to twin boys who shouted with delight in the background while she spoke on the phone yesterday with the Daily News.

"I have to admit it bothered me. I don't think the essence of a person comes down to the girth of that person. And I also think [Maya] thought I was too young, next to Mark," said Saldana, speaking of co-star Mark Ruffalo.

She laughs about it now, but at the time she was concerned and a little upset she might be passed over for "Polar Bear," based on Forbes' own life, the story of a mother (Saldana) who makes the wrenching decision to go to college, leaving her two daughters in the hands of her bipolar husband (Ruffalo).

"Most of the time when you're not right for a part, it's for some specific reason out of your control and you're not bothered by it. But I wanted this so badly. It resonated with me, it sang to me, it was very precious to me."

Mostly for the way it depicts the tender bond that develops between the girls and their father. Saldana took Forbes aside and explained how deeply personal the project was to her.

"It's hard to be that vulnerable because if you open up to that degree, and you're rejected, it really sucks. But I lost my dad very early on [at age 9], and any time I find a project that has a very beautiful and very real relationship between a young girl and her father - and this movie is just so perfectly accurate about that - I want to be part of that."

Forbes still wasn't convinced but yielded eventually to her own mother, who'd met Saldana and pronounced her "perfect."

All part of the strange process of how movies are made: Forbes, who is biracial, was initially told by studios to make the character of her African-American mother white. Forbes won that battle, Saldana won hers, and the actress is thrilled that she did. She describes "Infinitely Polar Bear" as the best working experience of her life.

"I never had favoritism about actors until I met Mark. He's the best actor I've ever worked with. I was so happy on that set, and so was my husband [Italian artist Marco Perego], because he loves Mark as well," she said.

"Mark's dedicated, but he never makes it seem like work. He's so good at his craft but very humble, like he's always sincerely worried that he's not quite good enough. He's the kind of actor I want to be when I grow up."

Saldana said it also was a nice change working for a female director, hastening to add that Forbes' skill, not her sex, made the shoot a pleasure.

Saldana, 37, is an accomplished Hollywood veteran ("Star Trek" Avatar" "Guardians of the Galaxy," all with sequels in the works) and like most actresses has a few war stories - like the infamous gun-and-panties guy.

"That story is true. A producer once told me he hired me for the way I held a gun while wearing panties, not for my opinions. I wish I'd recorded it, so I could play it for every girl in elementary school and tell them never to let anybody treat them that way. "

She worked for another woman, Cynthia Mort, for her Nina Simone biopic "Nina," a movie that is having a tough time getting to the screen. Right now, it's tied up in legal wrangling.

"We're still today struggling to finish the movie, to salvage it, and hopefully we'll be able to do that."

This was another passion project for Saldana. Like "Infinitely Polar Bear," it had a somewhat fraught casting process.

The role was offered to several singers (including Mary J. Blige) and actresses before it got to Saldana, who endured some criticism for being, in the minds of some, too light-skinned (she's Dominican, Haitian and Lebanese).

"I happened to be with Mary J. Blige in Paris, at an event, and before she got up to sing, she announced she was going to be playing Nina Simone. I cried because I was so happy, and she was so perfect," Saldana recalled.

"But that didn't work out, and a year later, the producers and directors met with me and offered me the role. For eight months I said no," said Saldana, who believed in the project but didn't think she was quite right for the wide range of ages she'd be required to play. Other actresses didn't like the script, or the budget, or the fact that it was an independent. The project started to look like an orphan.

"Finally, I thought, Nina deserves better. So we went for it. And no matter what happens, we tried and worked very hard, and we did it from the heart, from a pure place."

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