Will Smith (search) stopped saving the world long enough to make his romantic-comedy debut in "Hitch," (search) a movie about a Manhattan "date doctor" that's opening just in time for Valentine's Day.

"It's a comedy romance. You know, it's a film that is based in the harsh reality of New York City dating and trying to find love," Smith told FOX News.

In the movie, Smith plays Alex Hitchens, a professional date doctor trying to help a client woo his dream girl.

But the dating years were anything but easy for Smith.

"I had lots of trouble getting girls," Smith told FOX. "I grew up with a safety net though ... I had two sisters and I had a female friend who was my best friend, so I never actually had to put myself in a situation where I was approaching the woman. I always would send one of my sisters or one of my friends, then she would go over and coordinate it. So I would make sure somebody liked me before I made my move."

But despite growing up around all those women, he still finds the opposite sex confusing.

"Women fluctuate, you know? Woman at 9 a.m. is a completely different woman at 4 p.m. A guy that she'll ignore at 9 a.m., at 11 p.m. she might talk to him," Smith told FOX. "Guys just don't fluctuate like that, so we don't really understand.

Eva Mendes (search), who plays Smith's love interest in the film, agreed that women can be hard to understand.

"I think we're pretty much, we're crazy," Mendes told FOX. "[Guys have] got to put up with our hormonal imbalances."

As it turns out, the "Hitch" shoot in New York last summer brought up its share of relationship challenges in Smith's own marriage — though he and wife Jada Pinkett Smith (search) handled them with customary aplomb.

"I mean, wow, Eva Mendes is freaking gorgeous," Smith told the New York Post. And he insists he would say the same thing to Jada's face.

"In our marriage vows, we didn't say 'forsaking all others'" — though he insists he'd never cheat on Jada without asking her permission.

"If it came down to it, then one [spouse] can say to the other, 'Look, I need to have sex with somebody. I'm not going to if you don't approve of it — but please approve of it,'" he told the Post.

Ever since 1996's "Independence Day," Smith has been known for action-packed blockbusters ("Men in Black") that open in the heat of summer and make millions.

But now, the 36-year-old, $20-million-a-movie star is going back to his comedy roots — and some critics are disappointed.

“Will Smith waited a long time to bring his bottomless charisma to a romantic comedy," wrote Associated Press movie writer David Germain. "He should have waited longer, at least until the many hitches were ironed out of 'Hitch,' an occasionally cute trifle that had all the ingredients to be a great date flick.”

Germain gave the movie two stars out of four.

FOX News' Mike Waco, Trace Gallagher and the New York Post's Russell Scott Smith contributed to this report.