Parenthood's Dax Shepard brings the loves of his life - his fiancee, his friends and fast cars - to the big screen in his latest comedy.

Dax Shephard shines as a jack-of-all-trades in his directorial debut, Hit and Run, an R-rated action comedy he wrote for himself and real-life girlfriend, Kristen Bell.

The movie packs a punch with stunt driving (Shepard uses many of his own cars), an unrecognisable Bradley Cooper in a role that rids him of any physical appeal, and for some shock value, nudist senior citizens are thrown into the mix.

With Bell (best known as the title character in Forgetting Sarah Marshall) as his romantic interest, the film defies the notion that a real life couples equates to zero sexual chemistry onscreen.

Says 37-year-old Shepard: "I wasn't concerned about that 'rule' and I make an effort not to worry about how anything will be received, period. There are no rules. It's like when people say to comedians, 'Don't ever get in shape because that's not funny'. But look at Will Smith. He has a glorious body and he makes everyone laugh."


Directing one's other half might bring some challenges?

"Yes, it was a minefield, for sure," he acknowledges. "But I wouldn't have predicted we would have gotten along as well as we did. It was probably the best six weeks of our relationship."

Bell adds, "Neither of us wakes up in the morning saying, 'What would the public want from us?' Obviously, we hope people will want to see the film but that factor didn't go into the decision. We wanted to work together and that was that."

Shepard's career started out in Ashton Kutcher's prank show, Punk'd. He parlayed his success into comedic roles in films including Idiocracy, Cheaper by the Dozen, Baby Mama, and he then landed a regular role on the TV series, Parenthood, in 2010.

Shepard, as amiable in person as most of his onscreen characters, says, "I realised I was funny in the 6th grade. I was the go-to guy for laughs and that's when I realised I had a knack for it."

He put his politically incorrect humour to task in this fast-paced and somewhat unpredictable script, and to his credit, it came together in a brisk three weeks. Shepard wrote almost all the roles with his friends in mind; Bradley Cooper, Tom Arnold, and his Parenthood co-star, Joy Bryant. "I met Tom on an episode of Punk'd 10 years ago. I think he's the funniest guy in the world. Bradley and I had a mutual friend, Will Arnett, and then we worked together on a movie called The Comebacks. At that time, Brad was just a nice guy that I knew. He became a huge star while we were shooting this, which was very interesting," he recalls.

As for the older cast members, he says, "I'd written a scene for four seniors. We ran an advertisement, six responded and they all showed up. We thought, 'Well, to hell with it. And we put them all in.'

"Everyone had a great time and it was a fun day on set. In between takes, I'd say, 'You can put your clothes on now.' But they were like, 'Nope. I'm good.' They were bona fide nudists."

Says Bell:"Dax's humour is very politically incorrect. He's a liberal gun owner, he hosts gay marriage rallies, he's a thousand different contradictions. People are often a paradox. And to me, there's nothing funny or interesting about perfection."

The underlying theme of the movie is centred on Shepard's character, a participant in a robbery gone awry. He doesn't reveal his criminal past to his partner and problems arise in the relationship. This idea was inspired by Shepard's real life - he has reportedly been sober for seven years after addictions to alcohol and cocaine - and Bell says she had to adapt to dealing with her boyfriend's past.

"When I met Dax, he was my Prince Charming from that moment. Then I heard that he had somewhat of an exciting past. I immediately got nervous and felt threatened. I subconsciously punished him for things he did before he met me, which is really not ok.

"I had to recognise that I was being judgmental. It was something we dealt with in the first year of our relationship," she says.

Shepard's other passion - when he's not talking about his beloved effervescent blonde - is his love for cars, which is evident in the film.

"I'm from Detroit and I think all males from Detroit have an obsession with cars. My stepdad was an engineer in the Corvette group at General Motors, my dad sold cars and my mother owned a company that held press functions for General Motors and launched new vehicles. When I was 14-year-old, I started working for GM on race tracks and then worked for a race team. Eventually, I drag raced and I raced motorcycles," he says. "I guess you could definitely say that my number one hobby in life is automotive stuff."

Given his background, it's not surprising Shepard included several car chase sequences, many of which were inspired by some of his favourite movies.

"I love Bullitt, Vanishing Point, and The French Connection and, of course, Duel.

"Actually, I watched Duel before we shot this, but I didn't steal any of Spielberg's shots," he smiles. "Although I did steal some other shots."

Who: Dax Shepard and Kristen Bell
What: Hit and Run, also starring Bradley Cooper and Tom Arnold
When: Opens today