Patti LuPone is giving her radiator a good smack. “I can NOT figure out how the heat works here!” Lupone – the queen of Broadway, the goddess of the modern musical – bellows with the sort of fierceness that has driven people she’s worked with to hide in terror. “Is it hot or cold? I cannot figure out what’s going on in this country. I cannot figure what’s going on in my country, either. Oh my Gahhhhhd, don’t get me started!”

We are meeting in the London townhouse where she is staying while appearing in Marianne Elliott’s hugely anticipated production of the Stephen Sondheim musical, Company, which opens next week. LuPone is a theatrical legend, a two-times Tony winner who was the original Evita on Broadway and the original Fantine in the London Les Misérables, her distinctive, powerful and emotional voice helping to turn their accompanying albums into mega-sellers.

But LuPone gained an extra level of fame last year for her political opinions. In the summer, she was on the red carpet at the Tony awards when a reporter asked her why President Trump should come see her sing. “Well, I hope he doesn’t,” LuPone said, “because then I won’t perform.” “Really?” “REALLY.” “Tell me why,” the reporter said. “Because I hate the motherfucker, how’s that?” she replied. The interview immediately went viral.

“Ha! That was something!” LuPone cackles. “I can’t stand the red carpet. It’s just so tedious and I’m not good on it because I will always speak my truth, and it’s not necessarily appropriate in the environment.”

‘If someone has the talent, they have the RIGHT to be temperamental’ … LuPone. Photograph: Dan Kennedy

It is hard to imagine LuPone ever worrying too much about appropriateness. In the US, she is so famous she has played herself on Glee, Will & Grace and Girls, always three parts brassy, one part terrifying and two parts hilarious, which turns out to be pretty much what she’s like in real life. “And that is the last time she fucked with a woman in a wig cap,” was one of her lines on Girls, and it is easy to imagine LuPone saying that for real.

During our time together, she casually slays multiple reputations: “Hal Prince is the cruellest director I’ve EVER worked with! He’s a great stager, but a terrible director.” Prince is the 21 times Tony winner, who directed her in Evita in 1979. When I ask what she thought of Madonna’s performance as Evita, she replies: “Madonna? Euch! But I can’t really talk about Madonna. Madonna’s Madonna – I’ll leave it at that.” At one point, she describes an extremely famous person as mentally unstable, casually adding: “Maybe don’t print that because he once tried to sue me.” And as for Andrew Lloyd Webber, well, we’ll get back to him in a bit.

Doubtless directing LuPone presents its own challenges. “I am exacting,” she concedes with pride, “and I push.” But listening to her is a hoot. “If someone has the talent they have the RIGHT to be temperamental. They complained about Bette Midler when she was doing Dolly, but she wouldn’t be exciting if she wasn’t temperamental. It’s only the ones who don’t have the talent and are temperamental who make you say, ‘Just get out of here!’” Who would dare argue?

LuPone doesn’t just have high standards for herself and other performers, but also the audience. In 2009, she paused in the middle of performing Gypsy to call out an audience member for taking photos. In 2015, she descended from the stage and snatched a woman’s phone after LuPone saw her texting mid-performance. “I am fearless on stage. Not just to grab people’s phones – but I will do anything on stage, because anything can happen and the audience wants it to.”

‘I can’t get in trouble up here’ … LuPone as Evita in 1979. Photograph: Alamy

In her 2011 memoir, LuPone says she fell in love with acting at the age of four while in a school play. She realised: “Hey, they’re all smiling at me. I can’t get in trouble up here. I can do whatever I want and they’ll still smile at me.”

Unsurprisingly, rare is the article about LuPone that does not include the dreaded four-letter word “diva”. She twitches her head away at the mention of it, like she is flicking away a wasp. “It’s just because women are supposed to shut up, or not think those thoughts or talk back. It’s crazy. But talking back is something I’ve done since I was a little girl, and it’s always got me in trouble. Ha!”

Which brings us to Company, Sondheim’s 1970 musical about a young man called Bobby who just can’t settle down, much to his bemusement and frustration. But for this production Elliott has swapped Bobby for Bobbie, played by Rosalie Craig. “I think it makes the show more powerful,” says LuPone. “It’s almost cliched for a man to be noncommittal, and Bobby was always a conundrum for me. I understand the character much better now, because the stakes are higher for women who wait.” LuPone married her husband, a cameraman called Matthew Johnston, when she was 39. They have one son, Joshua, a writer and actor.

But it was the director’s gender that most intrigued LuPone: she hadn’t worked with a female director since 1977. “Which is insane. I mean, my GAHHHD.” So even though LuPone, now 69, had announced she would not be doing any more musicals, after a hip replacement, when Elliott offered her the role, she decided to postpone her semi-retirement. Was it worth it? “Oh, my God, yes!” she says, thumping the table. “We need more female directors and writers.”

‘The poor guy’ … LuPone and Andrew Lloyd Webber pre-feud in 1988. Photograph: Mario Suriani/AP

LuPone plays Joanne, Bobbie’s oldest and most cynical friend, who has some show-stopping numbers. As a role, it seems made for her, but when I ask whether she talked about it with Sondheim before coming to London she becomes uncharacteristically tentative. “Er, not with Steve, no. I don’t think I’m Steve’s first choice,” she says. But you’ve done so many of his musicals! “Well, that’s why. I’ve done so much by now.” I always imagined you two as having a close relationship? “Well, right NOW it’s good,” she says with a laugh. “We know each other socially, because we live near each other in Connecticut. But I have to be careful with him professionally, because he’s exacting.”

You’re pretty exacting, too. “I try to be, but I’m not as exacting as him with the score, and he’ll let me know when I’m not. And there were times when it was hard to take. I think I’m really lucky in that I call the man Steve, and I’m working with a living composer, who happens to be Stephen Sondheim, and I don’t try to take it further than that.” Watching LuPone speak with caution is a little like seeing an elephant tiptoe through a china shop and we both relax when she gets to the end without any disasters.

LuPone was born in Long Island, New York, the only daughter of first generation Italian immigrants. After her school introduction to the stage, she made it into the first-ever class of the Juilliard School’s drama division. Also in her class was a handsome curly-haired man called Kevin Kline, with whom LuPone quickly became “entrenched in love”. The two were together for seven years until, as LuPone bluntly puts it in her memoir, “he slept with a chorus girl in Boston”.

With more than a tiny amount of satisfaction, she says: “He was all over the place when we broke up.” Are they still friends? “Umm, we’re not enemies! We’re friendly, but friends are the people you call and see and hang out with, and very few of my friends are in show business.” After breaking up with Kline, LuPone was cast as the star of Andrew Lloyd Webber’s Evita, an experience so difficult she once said it was “like Beirut”.

No Manhattan transfer … LuPone in Sunset Boulevard, the show that sparked her feud with Lloyd Webber. Photograph: Rex/Shutterstock

“It was the thing that made me a star, but I had a terrible time. Oh my Gahhhhhd! That’s why I’m sort of counselling Rosalie. I know what it’s like to go through a trial by fire.” LuPone was so anxious about her singing, she practised until she lost her voice. During rehearsals, cast members tried to help by telling her how Elaine Paige had played Evita in London.

“Shut up!” LuPone eventually snapped. “And,” she writes in her memoir, “a reputation was born.” Because, while it’s true that a woman gets called a diva just for speaking up, it’s also true that LuPone’s memoir is studded with anecdotes about her telling people to go jump – and smashing mirrors in fury. “Why complain about someone who delivers?” she shrugs unapologetically, when I mention her reputation.

But Evita was a walk in the park compared with the next time she worked with Lloyd Webber. In 1993, he cast her as Norma Desmond in Sunset Boulevard in London, which led to what is still one of the most infamous feuds in theatre. LuPone had expected to take the show to Broadway, but Lloyd Webber fired her and replaced her with Glenn Close. LuPone sued him over their agreement and she got more than $1m, which she partly used to build a pool behind her family house in Connecticut. She gleefully told everyone it was called The Andrew Lloyd Webber Memorial Pool.

When LuPone performed Don’t Cry For Me, Argentina at the Grammys this year, it was widely assumed the feud was over, but such an assumption underestimates LuPone. “We haven’t made up, no. No! What he did…” Twisting the knife, she adds: “The poor guy – it seems to me he wants the kind of critical success Stephen Sondheim has.”

The Christian right in America is no different from al-Qaida

LuPone was also furious with Glenn Close. It probably didn’t help that Kline had a relationship with Close after he and LuPone broke up in the 1970s. I tell her that I read last year in a New York tabloid that the two women went out for drinks and made up, and were then joined by Jon Hamm, Mad Men’s Don Draper, and Girls star Andrew Rannells.

“That’s bullshit!” she says. “They got that ALL wrong. I was out with Andrew and Jon – and Glenn was at another table. Glenn came to us and she sidled on up to Jon Hamm. She needed to make herself known to Jon Hamm! Ha ha!” She laughs, casually plunging in another knife.

Despite all evidence to the contrary, LuPone insists she’s ready to slow down. She has an Italian passport (“which means I belong to 28 countries!” she says, sticking the Brexit knife into me) and she dreams of travelling “this beautiful Earth” with her husband. “I want the quiet life,” she sighs longingly, then immediately goes into a furious rant about how “the Christian right in America is no different from al-Qaida. Print that, because someone needs to say it out loud!”

And then, suspecting she’s just started another almighty row, she makes another big cackle.

• Company is in previews at the Gielgud theatre, London, and opens next week.