By James Mottram

An audience with Sean Penn is a rare thing. He's never been the most press-friendly of actors - put that down to his days fighting with paparazzi when he was married to Madonna. Even when he's been in films that have scored him Oscars for Best Actor - 2003's vengeful father tale Mystic River and Gus Van Sant's gay rights biopic Milk - he's never been one to press the flesh hugely. But here he is, dressed in a navy suit and white shirt, offering a firm handshake and a smile. It's disconcerting.

Even the journalist before me comes out singing his praises: "He's charming." If you want to go all tabloid about it, he's in the throes of love right now. The previous night Penn was seen on the red carpet in London for his new film The Gunman with his partner, actress Charlize Theron, on his arm. Then there are his humanitarian efforts, devoting time and energy to help out after disasters like Hurricane Katrina and the Haiti earthquake. Busy? "It's a seventeen hour day," he mumbles.

If it's all change for Penn personally, then his latest film is a shift professionally. Directed by Pierre Morel, who turned Liam Neeson into an ageing action star with Taken, this story of an assassin who takes out a politician in the Democratic Republic of Congo and lives to regret it was originally designed with Neeson in mind. But then in came Penn, who wound up as a co-writer and co-producer, as well as taking on the role of Jim Terrier opposite an impressive cast, including Javier Bardem, Mark Rylance and Idris Elba.

Set in a world of private contractors and governmental corruption, it's clear The Gunman is attempting to be more than just Taken, which for all its guilty pleasures was a straightforward thriller. "I didn't want this to be a thoughtless action picture.," says Penn. "And yet I didn't want it to become a pedantic thought piece either. I knew the nature of it - a high-octane genre. And so that's why Pierre was the perfect guy to come on board, because he's a thoughtful guy, interested in the world at large."

While the film's blend of politics and action is uneasy - and there are one too many shots of the 54 year-old Penn with his top off - it marks a fascinating pit-stop in the actor's career. He's rarely if ever ventured in this territory, even if films like At Close Range and Colors flirted with it. "I guess it was finding connective tissue," he shrugs. "There we plenty of action movies that I was offered to do and I just didn't see anything in them that was of any interest to me...I just responded to this one and I hadn't to others in the past."

So does this mean we're likely to see Penn 'do a Neeson' and start cranking out a slew of action movies? He shakes his head. Penn simply doesn't have that sort of conveyor belt mentality when it comes to acting. "For me it's essentially one more movie." he says. "I would say, I'm kind of an anything's possible person. I don't see myself becoming some kind of an action star, or anything like that. But I wouldn't walk away from a movie because it had a lot of exciting stuff in it."

Since completing The Gunman, Penn has gone on to direct his fifth film, The Last Face, with Bardem and Theron. Scripted by Erin Dignam, who wrote and directed Loved, a movie starring Penn's second ex-wife Robin Wright that he also briefly appeared in, it was actually Bardem that brought up the script during the making of The Gunman. "One thing led to another and eventually I directed it," says Penn, who calls the story, set amongst aid workers in Africa, "love in the time of conflict zones".

Given he devotes anywhere between two and ten years to developing films to direct, Penn admits that he's let the acting slide. Pre-The Gunman, his recent roles - the not-so-great Gangster Squad, a cameo in The Secret Life of Walter Mitty - would indicate a man out of love with the discipline. "I don't wake up in the morning looking for a new project as an actor. I haven't for years had anything that I was developing for myself as an actor. My constant is more on my other job and on things I might direct."

Perhaps it's no surprise. The Los Angeles-born Penn may come from a Hollywood family - his mother Eileen Ryan was an actress and his father Leo Penn an actor and writer-director - but he has always struggled with his enthusiasm for the business. "I've had a career of trying to muscle through disappointments and find out how much I really wanted to do it," he says. He was simply spoilt as a teenager, raised in the golden generation of 1970s Hollywood films. "The downside was that it could change pretty quickly."

He tells me he's just "shared" Scarecrow, the classic movie with Gene Hackman and Al Pacino, for the first time with his son, 20 year-old Hopper (Penn's second child with Robin Wright; they also have a daughter Dylan). "He's the youngest ultra-fan of Gene Hackman around now," he smiles. And when I ask if Penn is feeling content now, it seems fatherhood is a huge part of it. "Every day that I know my kids are happy and healthy...that's a big one." No wonder acting comes low down on his list of priorities.

The Gunman opens on March 20.