Sean Penn is an instinctive friend to the underdog: a champion of hard-fought causes and people who struggle against adversity - even when the trouble is of their own making. His infamous term of incarceration in LA County jail, way back in 1987, might be read as the root of such empathy. But prison wasn't really such an eye-opener for Penn. "A lot worse things have happened to a lot better people," he remarked ruefully in the aftermath. He has close friends who have served stiffer sentences, from a classmate in elementary school to a prominent Hell's Angel - not to speak of Andrew Daulton Lee, the convicted spy Penn impersonated so memorably in The Falcon and the Snowman (and later engaged as his personal assistant.)

When I spoke recently with Penn about his new films, The Interpreter and The Assassination of Richard Nixon, he was bemusedly reading a letter from a fan resident in a lock-down facility in Modesto, California. This inmate was encouraged to hope that Penn might like another prison pen-pal - such is the kind of correspondence you will get as a celebrity with well-advertised principles.

Then again, Penn's principles can rub some observers very firmly the wrong way - such as when he rose to rebut Chris Rock's vigorous mockery of Jude Law at this year's Oscars. Or when, in the countdown to he US presidential run-off of 2004, he upbraided the makers of Team America: World Police for their assertion that uninformed 19-year-olds would do better to abstain than cast a vote in ignorance. They are set-tos that serve to harden the perception of Penn as entirely without humour.

"Fun-loving" is certainly not the first adjective that most moviegoers associate with Penn. Too many hostile journalists have already consigned him to the ranks of the awkward squad. But ask Penn's pals to name his defining characteristic, and all will instantly cite his wicked wit. Woody Harrelson once told me cheerfully: "I've got a lot of fun friends, but it's hard to imagine having more fun than when Sean's having fun."

Right now Penn is on top of his game in every division. His work in 2003 - Mystic River, 21 Grams, a frontline opposition to the Iraq war, topped off by an Oscar - have been consolidated soundly by yet more first-rate work. Even the dogs in the street know by now that Penn is the foremost American actor of his day, the man who sets the standards by taking the boldest risks for the best reasons. He believes cinema can and should be a serious calling. Despite his occasional pained threats to quit, he is in the grip of that calling - and directors vie for his services. If the movies he does for love don't always work as well as some of the simpler stuff his contemporaries do for money . . . well, it's not for want of trying.

Take Niels Mueller's The Assassination of Richard Nixon, in which Penn plays true-life protagonist Sam Bicke, a sad-sack failed salesman and divorcee who starts to take Tricky Dick Nixon's clammy endurance in the White House as a personal insult. Mueller has praised Penn for sticking with this egregiously under-budgeted project since 1999, "difficult" material though it was. Watching the results I was reminded of how much Penn is a child of the Watergate era depicted therein, with all its fallen causes and craziness. But Penn insists his dedication was to Mueller's vision rather than memory lane. "Obviously it excites me when the writing is about something that I have an interest or passion for," he says. "But any great writing is passionate or it's not great writing. So, had you attached the crisis of this guy's divorce and broken dreams to other scapegoats or other triumphs - I would have been just as interested."

Others have seen Bicke as a mirror of Penn's antipathy to the current US president, but Penn himself is not of a mind that Nixon's own baleful influence has yet receded. He is still irked by the attacks made on John Kerry's presidential run by John O'Neill, prime mover of the group Swift Boat Veterans for Truth, who proclaimed Kerry unfit to be commander-in-chief. Like Kerry, O'Neill is a decorated Vietnam veteran. Unlike the senator, he never changed his mind about the rightness of America's cause, and so became a tool in Nixon's original counterassault on the veterans' protest movement of which Kerry had emerged as an eloquent voice. "Among the Nixon White House tapes," Penn says, still seething, "is the conversation that began the Swift Boat attacks on Kerry, back in 1971 - where they brought in John O'Neill and talked about using him to destroy Kerry. That had an enormous impact on the 2004 campaign."

This is the kind of skulduggery that gets Penn mad. Since Bush's re-election he has been one of many pro-Democrat actors and entertainers jeered as a "limousine liberal" by Republican columnists. But there is nothing armchair about Penn's politics. Throughout 2004 he was stumping for the shoestring campaign of the most leftward Democratic candidate, Ohio congressman Dennis Kucinich. Only when Kucinich withdrew did Penn put his shoulder behind Kerry's efforts.

In the final days before polling, Penn embarked on a whistle-stop tour of battleground states Arizona and Nevada, even canvassing door-to-door. The folks he met there had no quarrel with a well-paid actor exercising his political voice. "When you're one-on-one with somebody," Penn insists, "people have a basic decency and a belief that everybody's a citizen."

Whatever Penn's discontent with the current state of American politics, it should be noted that his second new release this month, Sydney Pollack's The Interpreter, is a politically hopeful enterprise: a thriller in praise of international justice. Nicole Kidman plays Silvia Broome, a United Nations linguist who suddenly becomes a lightning rod for all intrigues attendant upon the visit to the General Assembly of Edmond Zuwanie, Mugabe-like liberator/dictator of a fictional sub-Saharan state. Penn is Kidman's secret service guardian/interrogator.

The Interpreter unfolds at a fierce clip, 70-year-old Pollack directing like a man half his age. And Penn's own athletic performance reminded me of a remark I heard back in 2002 from James Foley, who directed him in At Close Range: "The one thing Sean hasn't done that I think he'd like to do, if he ever found a script that made sense, is an action sort-of-a-thing where he gets to run and jump and shoot people - because he could do it really well." The Interpreter is somewhat of this order, though Penn (who performs his own stunts) would gladly have worked even harder. "There could have been more running and jumping for my money," he says, laughing. "They've got a bomb-on-the-bus sequence, but I'm just standing there through it."

Penn was first drawn to this project by its affinities with the icy paranoid thrillers of 1970s Hollywood, including Pollack's own Three Days of the Condor. The Interpreter also has shades of Alan Pakula's Klute in Penn and Kidman's characters - their rhyming privacy and tentative togetherness. Meanwhile, the embattled UN emerges from The Interpreter in a more positive light. Penn, a vocal supporter of UN diplomacy throughout Bush's drive for regime change in Iraq, may also have found this aspect of the project appealing. As he puts it, "My position on the United Nations was and is that, if you give birth to a child, the child doesn't become any less valuable to you if he starts doing poorly in school, or even ends up in jail for a bit . . . You've still got to say, 'This is what we've got and we've got to find a way to love it and make it work."'

But there is scarcely an aspect of global politics that Penn cannot translate back into the family home that he shares with wife Robin Wright Penn and their two teenage children. As his comrade-in-arms Susan Sarandon says, "Sean's interest in politics is something that has grown concurrently with his children - the sense of responsibility for the world that they will claim." His next project is an outstanding choice in that line, as he partners Jude Law and Kate Winslet in a new screen version of Robert Penn Warren's All The King's Men, surely the greatest novel of American politics. Its anti-hero is one Willie Stark, a Louisiana governor modelled on demagogue Huey "Kingfish" Long, who fused ruthless backstage fixing with a populist pitch - "Every man a king, but no one wears a crown" - that resonates today.

Penn says of the Long/Stark figure: "If there was ever somebody who could convince enormous amounts of people that the ends justify the means - this might have been that person. So it's going to be very interesting to take what were the lessons of his time, and try to reapply them here, and see what people feel." This is authentic Penn - serious about how he does his work and what that work is for, desirous of a good audience and its honest opinion.