The premise of writer-director Ben Lewin's "The Sessions" is not a terribly promising one for a movie: a 36-year-old writer who was afflicted with polio as a child and who has to spend as many as 20 hours a day in an iron lung, decides he wants to lose his virginity, and enlists the aid of his parish priest and a sex surrogate to do the deed.

But the Polish-born Lewin, who is himself a polio survivor, has taken the tricky premise and somehow turned it into a funny, endearing little film that won audience and jury awards at Sundance and was immediately picked up by Fox Searchlight, which will release it on Friday.

The film is based on the true story of Mark O'Brien, who died in 1999.  Lewin enlisted the aid of both Cheryl Cohen Greene, the sex surrogate who helped him, and Susan Fernbach, O'Brien's girlfriend during the last years of his life.

A main reason the film works as well as it does is the acting, particularly the performances by John Hawkes as O'Brien and Helen Hunt as Greene. (William H. Macy also steals a few scenes as the priest, and the supporting cast is studded with able performances from disabled actors.) 

Hawkes, who is coming off back-to-back award-winning performances in "Winter's Bone" and "Martha Marcy May Marlene," spends the film contorted and helpless, restricted to a limited amount of movement in his head alone; Hunt, whose best-known film role was her Oscar-winning turn in "As Good As It Gets" 15 years ago, spends a good portion of the film in full-frontal nudity during sex scenes that are often tender, occasionally sad and usually funny. 

Hawkes and Hunt spoke to TheWrap about various aspects of the film, the real people they're playing, and all that sex.

First impressions:

John Hawkes: Humor was part of the appeal. It was a really interesting story, it was well-written, and the character was fascinating to me because of his humor.

When someone is ill-equipped to accomplish their goal, and they continue to battle without wallowing in self pity even though they have reason to do so, that's interesting and appealing. There's honor in that somehow, and something really human about it that strikes me.

Helen Hunt: A girlfriend of mine who never once in 30 years of friendship said "You should read this script" called and said, "You should read this script." And literally at the same moment my agent called and said, "You should read this script." So I did.

I loved it, and I met with Ben. You need to feel in good hands with a movie like this, and I didn't know that for sure. But I knew he wasn't creepy because I had lunch with him, and I met his family, and they were all lovely. And I knew that he was very well spoken and had an interesting take on this, and I knew he wrote something really good. That's more than you usually get.

The physical challenges:

Hawkes: I wasn't so concerned with playing the lead in a film with only 90-degree movement of my head. That was a little daunting, but the character was well drawn.

But it's mentioned in our script that Mark's spine is horribly curved, and in his autobiography, "How I Became a Human Being," he speaks very specifically about his body, head to toe, and how it all worked and didn't work. And Jessica Yu's short film "Breathing Lessons" [which features extensive footage of and interviews with O'Brien] was also an amazing tool at my disposal – between reading, Mark talking about his body, and watching him in Jessica's film, there was a great template for how to approach Mark physically.

So I conceived of and helped devise a soccer-sized ball of foam, which was placed midway under the left side of my back throughout every take. Knowing that there would be no body double or CGI or prosthetics or makeup, I needed to approximate that curvature.

The ball immediately gave my back a huge arch. And then each limb was doing something different.  Finding that contorted position was difficult, and it often hurt, but holding that position for long periods was the real challenge.

Mark is not just a head sticking out of an iron lung. You see his body. Nearly every inch of it.

Speaking of which … the nudity.

Hunt: "I needed to work out how the nudity would be filmed, so I got together with the cameraman and with Ben. It was all about, "I'll lie here and he'll lie here and the camera will be here," because I needed to cross that off my list of worries.

You cross off anything you can control. Anything beyond that is just my own troubles. My character's job is to bring mastery and confidence -- or at least a good proximity of confidence -- for his sake. What I couldn't do is feel defensive about being naked. That wouldn't work. That would have been exactly the wrong mindset.

Hawkes: My character was hung up. When Helen Hunt reads the script, she says, "Oh, it seems to say that my character is very comfortable with her body," so a great actress, as she is, does that, and becomes very comfortable with her body, at least for the scenes that we're shooting.

For me, modesty would make more sense. Any nervousness I had about my own body, and about being naked in front of a cast and crew, I was able to use. I didn't have to wish it away or think it out of my system. I it used in my work, which is easier than acting.

The details they got from the real characters they're playing:

Hawkes: I like detail as an actor, I like specificity. I feel like the more detailed you can be when you tell a story, the more universal it becomes. And I didn't have to make up a lot of things here. He died in 1999, but I had his writing and I had Jessica Yu's movie – there was Mark, body and voice on the screen. And I could learn a lot from that and try to emulate him. I wanted people who knew Mark to see as much of their relative or friend in what I was doing as possible.

Hunt: A lot of times they put you together with the real person and it's not so helpful, because you've already planned a lot. But in this case, talking to her got me completely fired up about the part. I didn't really see what a great part it could be until I met with her. Her volume level and her enthusiasm and her adventurousness about talking about the cup of tea or her granddaughter or the orgasm or the movie was so infectious. And the idea of bringing that attitude to sex seemed like a movie I had not seen before. So I got really excited.

More about sex:

Hunt: My conception of sex surrogates changed from nothing to knowing a lot. I had never heard of such a thing. But she's taking one of the most precious parts of life, which is corrupted in little and big ways all the time, and giving it to this man. He described sex as standing on the wrong side of the glass watching a banquet that he would never get to taste. She gave him, and his future partner, a seat at that banquet. That'd amazing.

She also used the term sex positive, and I got all fired up about that. I want to be sex positive. I want to be part of making a movie that is sex positive.

The roll that Hawkes has been on, from "Winter's Bone" to "Martha Marcy May Marlene" to "The Sessions":

Hawkes: One really great byproduct is that these three films are independent films, so I'm getting a lot of really great independent films being sent my way. The issue with that is saying yes to some of these great movies and then spending the last nine months not working, because it's very difficult to make an independent film these days.

I'm actually working less at this point than I have for a while. It's partly by choice – I've turned down some larger-budget things because I wasn't sure if I wanted to be on a TV show, and I thought the larger-budget movie would get in the way of something I already said yes to, even though that didn't end up happening. So there's been a lot of "one step forward, two steps back" with the process lately.

Also read: Why John Hawkes Turned Down 'Walking Dead' [6]

But I'm at the point where I don't get to do every role that I want to do, but I don't have to do any roles that I don't want to do. And that's a good thing.

The moral of the story (which is about, you guessed it, sex):

Hunt: It'd be so great if a lot of people saw this movie, and a lot of people brought their teenagers to see this movie. I think that would be pretty amazing.

So many people my age say, "My parents never even talked to me about sex." I'm in a community of parents that are trying to figure out how much to say, when to say it, and how can we leave our own weirdness at the door? And I want to channel this lady when I have that conversation with my kids.

I want to tell them that they don't have to tolerate anything. That they can say what they want and what they don't want. That sex can be celebratory, and silly, and imperfect. That would be a lot, if they can get all that.