Recently, Martin Sheen has taken to keeping a tape of Hail To The Chief in his pocket and, when driven around the Warner Bros lot in Burbank, California, he slips it into the shuttle van's player, winds down the window, and waves magisterially to anyone who happens to be passing.
It's not that heading the hottest show on US television has gone to his head, it's more that he's having a ball and he wants everyone to know about it. "I'm having the time of my life," he beams.
We're sitting in the "Oval Office", Sheen's current place of business where he plays President Josiah Bartlet of The West Wing, a fact which clearly still tickles the hell out of him. And so it should: with nine Emmys and some 20 million viewers, The West Wing - which has been on RTE and started this week on Channel 4 - is the best thing that could have happened to the 60-year-old actor, still best-known for performances in Badlands, and Apocalypse Now, over two decades ago.
But a populist drama series set behind the scenes at the White House wouldn't be on everyone's most-likely-to-succeed list, and, given the show largely consists of interior dialogue scenes about the political process, it's a pretty good indication of how sharp the scripts are that it has taken off so unequivocally.
For Sheen though, the possibility of addressing the social issues he regards as vitally important - poverty, alcoholism and gun control, among many others - to a potentially huge audience was always going to be hard to resist: "It's not an accident I got this part," he suggests. "If Bartlet had been a Republican, you wouldn't see me sitting here, I promise you."
A reformed alcoholic and drug abuser, Sheen has devoted his life to the causes which his hatred of injustice and deeply held Catholic faith draw him to. He's known for flying off at the first sign of trouble to put himself on the line for what he believes in; he has been arrested more than 70 times for the causes he supports.
His engagement with grassroots politics may give Warners a mild headache, but it has added a certain frisson to an already highly-charged show, and anyway, Sheen is not about to put his life on hold for them: "They've never gotten in my face about my activism, and it would be a very serious mistake if they were to do so," he says. "They knew who I was, and what I stood for, when they offered me the job."
Despite the protestations of writer Aaron Sorkin and producer John Wells (also of hospital drama series ER), Sheen is quick to see similarities between Bartlet and the outgoing president, one of his heroes, Bill Clinton.
"Bartlet is a very intelligent man, with great heart and a great sense of humour who plays fair and enjoys being the president. I think all of these qualities are reflective of Clinton, who I think history will reveal as a great president."
He has little sympathy with the post-Lewinsky critics: "I think it should have been just the opposite - rather than condemnation for his sexuality, we should have celebrated that he could accomplish such great things, and be so human. It's a sad reflection of our country that we still haven't dealt with our sexuality."
Despite his enthusiasm for the project, Sheen is not without criticism; he is greatly frustrated by Bartlet's willingness to bomb the Middle East at any opportunity, and he remains unsure of the rosy glow in which the president is always framed.
"We can be too sentimental, but then I think that is how the office is generally seen and, God save us, presidents do become sentimental," he laughs, adding, "we had an old actor in there who was precious little else."
The biggest single criticism of The West Wing - a show which has hooked US liberals and conservatives alike - is that Bartlet is just too good to be true. "I agree," he whispers conspiratorially.
Sheen's politics are inseparable from his religion: "Your faith has to cost you something, otherwise you have to question its value. I do the things I do because I cannot not do them and feel good about being human. Our faith, our religion, has to connect us to our humanity because that's how we find ourselves in others. I just try and be as human and as giving as I can be and know that it will never be enough."
When I suggest that he's sounding too good to be true himself, he lets out a peal of laughter before offering a different analysis: "My faults are known to those closest to me, are rumoured by those furthest away, and are confirmed to my confessor, where they belong."
The first series of the The West Wing is on Channel 4 on Thursdays. The second series begins on RTE 1 in late spring.