George Clooney opens the door of the Berlin hotel lounge and shakes hands like an ambassador. “Come on in,” says this paragon of modern Hollywood: a proper, old-fashioned movie star; a producer and occasionally director of interesting, intelligent films; and a furrowed-brow liberal political activist of not inconsiderable achievement. Who else would spend the morning after the premiere of his new film, the Coen brothers’ Hail, Caesar!, confabbing with Angela Merkel about the international refugee crisis? He should be running for president, surely?

Clooney chuckles indulgently. “I am a Hillary supporter. I am doing a fundraiser for her.” That’s a big endorsement; Clooney’s 2012 event for Obama raised more than $12m (£8.5m) in a single night. But he has conciliatory words for her main Democrat opponent. “I really love Bernie Sanders, and am really glad he is in the debate. He is forcing the conversation to things that never get talked about in US politics: disparity between the rich and the poor, which is getting worse and worse every day.” He says he admires Sanders’ singlemindedness on the topic, but suggests it is the same character trait that is his “downfall” on the national stage.

Donald Trump’s increasingly likely elevation to Republican candidate is, he says, crazy; but course, being Clooney, he has had words with and been personally insulted by the other side. “I met Donald once. I was sitting in a booth and we talked for a while. Then he did Larry King, and he said I was very short. And I was like: ‘I was sitting down the whole time, Donald!”

However, Clooney’s geniality doesn’t extend to Trump’s political aspirations. “He’s just an opportunist. Now he’s a fascist; a xenophobic fascist.” But he is not entirely despondent. He repeats the old saw, usually ascribed to Winston Churchill: “You can count on Americans to do the right thing after they’ve exhausted all the other possibilities.”

“Let’s put things into perspective. You know, the truth of the matter is, in election season, things go crazy, and the loudest voices are the furthest and most extreme. So you hear a massively stupid idea, like we’re going to ban Muslims from the country. Now, we’re not ever going to do that.” He shrugs. “It says in the Statue of Liberty, bring us your huddled masses. It’s not what’s going to happen.”

As Clooney says, it is election season, so we can forgive him a little preoccupation. Anyhow, he doesn’t really need to be president. He says himself: “I enjoy my life, and I think most of the people who know me think I am pretty positive and have a funny sense of how life is.” Having so thoroughly conquered the movie industry, doing a medium-sized role in a new Coen film seems like a frivolous diversion, or a mild inconvenience: a busman’s holiday. Plus, he is a film star playing a film star, a gormless, 1950s version of himself, in a film that is partly about the surreal production-line nature of Hollywood’s golden age. His role, Baird Whitlock, has been described – by the Coens themselves – as part of a “numbskull trilogy,” or more accurately, a tetralogy, considering this is the fourth time that Clooney has played some kind of dope for them.

Clooney explains how Joel Coen told him to play his first numbskull, O Brother, Where Art Thou?’s Ulysses Everett McGill, as “the smartest guy in the room – all the time”. He followed it up with divorce lawyer Miles Massey in Intolerable Cruelty and trigger-happy US marshal Harry Pfarrer in Burn After Reading. “So I’ve played all of my idiots with them like that.” But, he adds a little sadly: “I don’t really think of them as idiots, although everyone else seems to.”

Still, Clooney says he owes the Coens – and specifically O Brother – a lot. He launches into an impressively concise summary of his own career, from the hit TV series ER (“people forget the size of it, we were averaging 40 million, 45 million a week, just in the US”) through his difficult entry to film (“it was very hard to translate from TV at that point; very few people had”) and his early stumble with Batman & Robin after it looked like From Dusk Till Dawn had set him on his way. (“Who would turn down playing Batman? And I wasn’t very good. It’s a fair deal all the way round.”)

His late-90s “comeback” in Out of Sight and Three Kings wasn’t, it turns out, the unalloyed triumphal procession it may seem from this distance. “You gotta remember, it’s an interesting thing how time changes the perspective. They were really good movies; but see, they didn’t do well. So I was coming to the end of my five-year contract on ER, and I was going to leave the show and do movies, and everywhere you looked, they said, is he going to make it? Is he going to be a movie star? Are they even going to let him do movies? Then the same year as O Brother, The Perfect Storm came out. Perfect Storm is a gigantic movie about a gigantic wave. It has very little to do with me. But it was a hit. And since I took all the shit for Batman & Robin, which had very little to do with me either, I thought: ‘Oh yeah, I’ll take all the credit for that.’”

Clooney with Chris O’Donnell as Batman. Photograph: Allstar/WARNER BROS

“So the one-two punch of O Brother … and Perfect Storm changed everything for me, and put me in the position where I was going to be able to survive in the film world.”

And survive he did. Aside from his acting, Clooney has directed a handful of substantial films – admittedly of varying quality – and produced a couple of dozen more. Good Night, and Good Luck, his paean to witchhunt-breaking TV broadcaster Ed Murrow, is arguably his most personal; he says he did it as he “was being called a traitor to my country because I was against the Iraq war”. With his politician’s charm – fuelled by an inexhaustible supply of silver-fox suaveness – Clooney gets on to the subject of diversity, a topic still convulsing Hollywood. He takes himself to task a little: “One film I did, The Ides of March, where the lead was Ryan Gosling. And yes, that’s one I didn’t think to, and should have probably thought to, look for a young African American actor to do it.”

But as he is fond of repeating, the practicalities of film financing are a brake on change. “The studios give you a list of five names on a piece of paper. If you don’t put one of those names in your film, it doesn’t get financed. It doesn’t matter what I’m doing; that’s the list of names. I’d like to see that list change a little bit, especially as, more and more, the star system isn’t filling seats.”

“I’ll say this honestly: I wasn’t paying attention to it at all. I thought we’d ticked that box, and it feels like we’ve slipped, considerably. It’s something we have to pay attention to.”

With David Strathairn in Good Night, and Good Luck. Photograph: Allstar/WARNER BROS

Even if he has the slightly disconcerting habit of referring to Hollywood as “we”, Clooney makes a fantastically relaxed interviewee – “I grew up in and around the press, so I don’t hate them, and I don’t panic around them” – but with a true pol’s instinct he can’t resist bending it to his programme. “If you watch Good Night, and Good Luck, you’ll see I have real respect for the profession. but I also get really ticked off when it’s used stupidly.”

“Right now we are dealing so much with domestic politics that no one talks about real world issues. No one is talking about the Syrian refugee thing in the US – nobody. You might hear little snippets of it, five seconds on the news. It’s a big worldwide issue, and it needs to be talked about. If the press covered it more, we would be more involved, and do more.”

The high sign arrives; the interview is over. Ever gracious, Clooney leaps to his feet and sees me to the door. If I’d been holding a baby, I think he’d have kissed it. And with that, it’s time to leave.

Hail, Caesar! is released in the UK on 4 March