"I didn't keep track of the days, but I put a little time in to try and see what it might be like to be this character right when we started filming," confided the handsome 23-year-old. "It's always something. You have to prepare in different ways."
Filmed after "Pearl Harbor," but before "Black Hawk Down," Hartnett said he enjoyed the break from serious drama that "40 Days" afforded him.
An actor who made his name with memorable performances in little-seen, but critically acclaimed films like "The Virgin Suicides" and "O," Hartnett more than holds his own in big budget flicks, as well.
Speaking of the runaway success of "Pearl Harbor" and "Black Hawk Down," both of which were earned Academy Award nominations earlier this week, Hartnett said, "I'm always surprised when a movie I'm in gets seen by a lot of people."
"40 Days" is his first big-screen attempt at broad comedy. He plays a young man who is so disenchanted with love that he opts to abstain from sex for 40 days so he can figure out what he wants in a woman. Of course, the perfect girl shows up at this point, making his vow much tougher to keep.
Hartnett said he feels lucky to be able to move from sweeping romance to combat drama to side-splitting comedy, noting this freedom is probably possible because after 10 movies, directors and audiences still haven't quite defined who he is or should be yet.
"That allows me to grow," he said. "It allows me to find myself."
Asked if he was embarrassed by the movie's over-the-top sex scenes, Hartnett replied: "To tell you the truth, while we were shooting it, I didn't find it at all embarrassing. Watching it a little bit, I guess. That's kind of the point of getting into a character. This situation means a lot to him, so you don't really think about it as embarrassing at the time, but then when you see it, it's embarrassing. Yeah, certain things, if I had any common sense, I'd take back. But I don't."
Down-to-earth and immensely likable, Hartnett recalled how he laughed out loud when he first read the script.
"It came across my desk. Desk? I don't have a desk," he admitted. "It came across my couch one day and I read the script and I laughed really hard... I didn't think they'd ever make it because even the script is so hyper-sexual I didn't think we were to that point in the studio system. I didn't think they'd go ahead and make a movie that was so sexually charged and on a real level."
Hartnett said his mother hasn't seen the film yet and he's not rushing to show it to her.
Since the movie is basically a love story, Hartnett was, of course, needled by reporters to discuss his own romantic life.
"I think love is ever-evolving," he explained diplomatically. "I don't think there's any one way to really tell [you're in love.] I thought I've been in love before and I think I have been."
So, is he in love now?
"Yeah, I think so. I feel like I am. But it'll keep changing," he said. Ever the gentleman, Hartnett said he didn't want to discuss his significant other further since she didn't choose to be in the spotlight.
After a bad break-up, the heartthrob admitted he likes to "kind of wallow in misery."
"Well, I'm sometimes kind of a dramatic guy. I guess I kind of sometimes... I'll maybe sit there a little too long and think about the possibilities, 'Why doesn't she love me?'" he explained.
The actor also said he tries not to get caught up in the trappings of wealth and success, instead choosing to surround himself with people who are honest and have integrity. He added that after five years in show business, his "bull-s*** meter" is getting more fine-tuned.
Hartnett admitted that he is far from perfect, occasionally giving in to distractions and temptations, maintaining that "it's all part of the ride all part of life." He insisted, however, that it's a stable life and not "floating around from club to club and girl to girl" that he really wants. Pointing out how "finicky" the movie business is, he said he is afraid that if he lives his life in fleeting manner, he'd have no base to hold on to should his career ever fade.
Having moved back to his native Minnesota after a year in Los Angeles, Hartnett described himself as "more of a Minneapolis-New York kind of guy." Stating that he can totally understand why his friends love LA, Hartnett said: "When I'm in a city, I want to see it. I don't want to see it over there." He added that he is also a big fan of "proper seasons, winter sports and snow on Christmas."
"40 Days and 40 Nights" opens March 1.