George Clooney is having fun and for once no beer or blondes are involved. The actor once dubbed the sexiest man alive is steadily shedding his hard-earned reputation as one of Hollywood’s most dedicated bon viveurs.

Clooney, it turns out, has a serious side, soon to be on worldwide display in a pair of new films that have earned rapturous praise in America.

Like Warren Beatty and Mel Gibson before him, Clooney is transforming a career as a Hollywood hunk into a fascinating second turn as a director and writer. He is also emerging as a successor to Beatty as the outspoken guardian of Hollywood’s liberal conscience and as a political activist prepared to lend his name to a range of causes.

If that all sounds a bit heavy, don’t worry — Clooney has not lost the sense of fun that made him Hollywood’s most feared practical joker. He once gave a friend a painting he’d found in a dustbin, pretending to have painted it himself. The picture took pride of place in the friend’s living room for two years until Clooney finally confessed. Other victims have included Brad Pitt and Julia Roberts.

Lately, though, the jokes have been giving way to political bromide. Earlier this year he joined Bono in the Make Poverty History campaign, appeared at the G8 summit in Scotland, joined marches in London and Berlin and, more recently, interceded with the King of Morocco to help to secure the release of John Packwood, a British yachtsman jailed without trial on false drug charges.

Somehow he also found time to advance his Hollywood career: Clooney, 44, wrote and directed Good Night and Good Luck, a new account of journalistic bravery amid the communist witch-hunts of the 1950s McCarthy era in America. He also stars as a distinctly unglamorous CIA agent in Syriana, a thriller about Middle Eastern oil. Both films might earn him Oscar nominations.

“It’s been a good year,” says Clooney. “It’s actually nice now to sit down at publicity junkets. Usually it’s entertainment reporters and their first question is, ‘Okay, who are you dating?’ ”

The former heart-throb of the hospital series ER still fields regular queries about his female companions — there do seem to be rather a lot of them — but most questions these days are likely to be about the Iraq war, presidential politics, corporate mischief and civil rights activism.

I caught up with him as a limousine whisked him out of Los Angeles to shoot the finishing scenes to his next film, The Good German, a dark political thriller set at the beginning of the cold war.

Clooney was far from the only Hollywood star to criticise the Bush administration’s strategy on Iraq, but the conservative establishment rounded on him particularly viciously. For a few weeks in early 2003 he was savaged daily by chat show hosts. He was scornfully portrayed by Bush supporters as a Hollywood airhead who was not only ignorant of the issues involved but was also somehow jeopardising American security by daring to disagree with the president.

“All I had done at that point was say, ‘Well, I think we have some questions to ask before we send 150,000 kids to get shot at’,” Clooney says. He also suggested that it was important to understand what motivated the terrorists. “Suddenly I was on the cover of one magazine: they called me a traitor. They had packs of ‘traitor’ playing cards going round and I was the Queen of Hearts.”

At one point a chat show host asked him if he thought his career was over because of his political views. “I was right in the middle of getting beaten up good,” says Clooney. “Bush was saying that you’re either with us, or you’re with the enemy. So to disagree was actually an act of treason.” Far from discouraging him, the experience convinced him that he had to keep speaking out.