Sean Penn has gone all Zen master. “We ask: ‘What is the meaning of life?’, when we should say: ‘What is the meaning of death?’ ” ponders the 47-year-old Tinseltown icon, former irascible paparazzi-puncher and current top-flight movie director. “Because only in our death do we make sense of our life,” he continues, chugging on a cigarette. “And that’s why the traditional paradigm is so wrong.”

If Penn seems unduly concerned with interrogating the essential philosophical nature of Man, it’s partly because his new directorial outing, Into the Wild, does just that. The movie, a ten-years-in-the-making passion project, is the true-life account of a rich and disaffected college kid, Christopher McCandless (played by Emile Hirsch, pictured being filmed by Penn below), who exchanged white-collar careerist ambitions for a penniless two-year trek around America and whose adventures culminated in an illuminating yet brutal climax, alone and starving in the Alaskan wilderness. Easily Penn’s most accomplished film to date, it is a spectacular portrait of natural America that pushes eye-gouging landscape cinematography to its limits. But it’s also a deeply moving account of idealism unbound that will leave you reeling at the fate of McCandless in the film’s final transcendent frames.

More importantly, however, the movie will finally confer on Penn the status of a heavyweight Alist director and confound those critics who saw in previous Penn efforts, such as The Pledge and The Crossing Guard, merely a brooding extension of the actor’s simmering tough-guy persona. “There is certainly a bias that is held that way about the films I’ve made before,” agrees Penn, slouched in a huge yellow armchair in a plush hotel in Toronto, where Into the Wild is receiving its premiere at the city’s film festival. “I suppose I have given in to that bias with the projects I’ve done before. But what I’ve always had inside me, and what I’ve hungered to do in film, is much closer to this than anything else I’ve done as a director or as an actor.”

Surprising too for the Penn-watchers out there might be Into the Wild’s blatant national pride. Penn, who burst on to the scene in the 1980s as a hot-headed actor in movies such as At Close Rangeand Casualties of War and then as the hot-headed husband of Madonna, is today undoubtedly the Hollywood actor most vilified by the American Right. His recent trip to Hugo Chavez’s Caracas, following much-publicised visits to Iraq and Iran, and regularly printed broadsides against the current Republican administration, have done nothing to lessen his image as a seditious pinko traitor.

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And yet Into the Wild, with its generous depiction of ravishing natural landscapes filled with kindly caring people, is nothing less than a reinvented image of modern America, or at least a heartfelt reappraisal from one of its prodigal sons.

“Is this my love letter to America?” Penn asks himself.

“Yes,” he replies. “Will people be surprised by it? If they’ve misled themselves in the past, yes. Do I consider myself, and always have considered myself, patriotic? Yes. I just don’t think that engaging in the blatant criminality and stupidity of this particular Administration defines patriotism.”

He says that he became an ultrapolitical animal only after the birth, with his second wife Robin Wright, of his two children Hopper (now 14) and Dylan (16). “Because then something mattered more. The future became something real and present, and if I didn’t participate in it, then I’d have really let them down.”

Mostly, however, he speaks with messianic zeal about Into the Wild, and about how transformed he feels to have finally found his metier as a movie-maker. “I now know what’s at the heart of my desire to make films and tell stories.”

And if all this talk of directing seems strange from an acting legend, Penn explains that he’s always been a director first and an actor second. “I directed little Super8 movies in school,” he says. “I’d shoot at night, but all the other kids would be doing their homework, so I had to act in my own movies. And, eventually, I started to like it.”

The roles followed – bit parts in Little House on the Prairie, and a scene stealer in Fast Times at Ridgemont High (he created the teen stoner archetype as the drawling surfer Jeff Spicoli). His professional promise in the 1980s was often bedevilled by a hair-trigger temper that made tabloid headlines (his marriage to Madonna was, typically, “tempestuous”) and eventually landed him in Los Angeles County Jail for 32 days (for assaulting an extra on the movie Colors). The 1990s, however, were kinder, and the roles got meatier, the Oscar nominations started rolling in (for the likes of Dead Man Walking and Sweet and Lowdown), and eventually a win for Mystic River saw Penn unequivocally labelled the Greatest Living Screen Actor.

For now, though, Penn says that his acting, his film-making and his politics are all merging into one obsession, which is why Into the Wild might just be the greatest and most pure expression of Sean Penn-ness yet. “[The novelist E. L.] Doctorow said the greatest responsibility of the artist is to understand the times in which he lives,” he says. “I think that’s true. It’s all the same project. I’m getting a clearer and clearer sense of what that is.”

Does that mean that, finally, the troubled firebrand Penn is calm and collected about his future? Or is he cynical about the possibility of changing anything? “I’m not cynical,” he says. “I’m p***ed off, I’m heartbroken, but I’m not cynical. A friend of mine once wrote: ‘We all know deep down in our heart this will all fall apart, so for right now let’s just be heroes.’ Well, it seems like, for me, it’s either that or eat the gun.”

He takes a short drag of his cigarette, and adds, with trademark macho aplomb, “And I’m not interested in eating the gun.”

Into the Wild shows at OWE2, Oct 25, 8.30pm; Oct 26, 12.30pm. It goes on general release on Nov 9

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