‘I hate watching my plays,” Tony Kushner says. “They’re all so long and they take for ever and the only one that’s not true of is Caroline. It’s two hours with intermission and I could listen to that music for ever.”

Caroline is Caroline, Or Change, Kushner’s only musical. Written with the composer Jeanine Tesori (who wrote the music for Fun Home) in 2003, it won an Olivier award in 2007 and it’s now enjoying a run in London’s West End. Caroline, the African-American maid at its centre, spends most of her days in a basement, vibrating to the rhythms of the radio, the washing machine, and her own sorrowing heart. Kushner works from a basement, too, in a room beneath an apartment building in Manhattan’s East Village.

Heaving bookshelves are the defining feature of Kushner’s basement. They cling to every available wall and scrape the high ceilings. There are books on theatre, religion, psychoanalysis and many of the spines are dotted with colour-coded stickers, remnants of an organisational project that manifestly failed. Kushner sits beneath those spines, two days after Thanksgiving. He and his husband, the writer Mark Harris, prepared a holiday dinner – turkey, a salad, plus two kinds of stuffing, two kinds of pie, two kinds of ice cream, because even his meals are dialectical – for 12 friends, “cooking and cleaning into a complete state of exhaustion”, he says.

For $30 a week … Caroline Thibodeaux as a black maid in a 2012 production of Caroline, Or Change. Photograph: Vince Talotta/Toronto Star via Getty Images

Kushner is still recovering. A fuchsia scarf hugs his neck; his voice rasps. He talks anyway. No surprise. Oskar Eustis, the artistic director of New York’s Public Theater and a longtime collaborator, has described him as one of the great talkers of the western world. Wry, philosophical, plain mesmerising, Kushner, it seems, can talk his way through anything. Caroline is a favourite subject. And his favourite show. Take that, Angels in America.

Caroline tells such an uncomplicated story, it’s almost a parable. In 1963, in Louisiana, in an assimilated Jewish household much like the one Kushner grew up in, Caroline labours for $30 per week. The family’s young son, Noah, carelessly leaves loose change in his pockets. To teach him a lesson, Noah’s stepmother tells Caroline that she can keep what money she finds.

Simple enough. But there’s great complexity in Tesori’s deployment of popular and classical musical forms and the way in which Kushner allows larger social concerns to penetrate acutely personal struggles. When director Michael Longhurst’s revival, which stars Sharon D Clarke, premiered last year at the Chichester Festival theatre, the Observer’s Susannah Clapp called it “one of the most gloriously disruptive, completely distinctive musicals of the past 20 years”.

Kushner talked with Longhurst about American Jewry and 1960s Louisiana, shared some old photos, sat in on a run-through. He and Tesori saw the full production in Chichester last year and he gave Longhurst only one note, unprecedented for Kushner, who is an inveterate note-giver. The note was about a wig.

Lesson … the maid Caroline and Noah, the son of her employers. Photograph: Marc Brenner

In Caroline, the civil rights movement is the starch that stiffens the script. The musical was written before Barack Obama’s presidency and it’s being revived afterwards, when civil rights gains seem less assured. But while Kushner acknowledges that “it’s very dangerous where we are right now,” he still has confidence in a union becoming more perfect, more equal. The midterm results have heartened him. “As long as we are vigilant about rejecting demagoguery, even demagoguery of the left, and re-engage with the democratic process and the electoral process, this” – by which I think he means the US – “should work.”

If you know Kushner’s work, which also includes the screenplays for the films Lincoln and Munich, you’ll know him as a political playwright, though he’s careful to draw a distinction between art and activism. Which makes him feel a little guilty. “Because I’m a Brechtian at heart,” he says. But he believes that theatre shouldn’t lecture, shouldn’t propagandise. “If you try to go at it too directly and give people their marching orders it’s gonna be bad,” he says.

Still, he doesn’t believe there’s such a thing as a nonpolitical play. “I sometimes wonder if I would write a play where the political is almost never mentioned,” he says. “In a certain way, it’s Caroline. But no, it’s very much there.” He says he doesn’t know how to think about the world “without philosophy and political theory and history and psychoanalytic theory. I feel like the epistemological frameworks I’m familiar with are necessary and I need a library to come at the world.” As he speaks, his own library towers above him.

Denise Gough and Andrew Garfield in the 2017 London revival of Angels in America. Photograph: Helen Maybanks

If you take all those -isms and –ologies in one hand and a refusal to propagandise in the other, and complicate the pose with the belief that theatre also has a duty to entertain, then you will realise that the writing of a Kushner play involves a very cerebral kind of juggling act. A show such as Caroline sinks the political down down down in the personal, but Kushner is often at pains to maintain, he says, “a tricky balance between wanting a play to address every issue with great complexity and not turning it into something that nobody can follow.” He has published the scripts for all his plays except for The Intelligent Homosexual’s Guide to Capitalism and Socialism with a Key to the Scriptures, because he worries that its arguments overwhelm the characters.

Kushner has several projects in the offing, some a little more political and some less. (Manuscripts litter the table between us and it takes great willpower not to peek — well, not to peek too obviously or too often). He’s on his fourth draft of a new film adaptation of West Side Story, which Steven Spielberg is now casting. Kushner won’t say much about it, except to note that he wants to avoid any suggestion that he and Spielberg are “trying to fix something that was broken”. The Broadway version is a great work of art, the 1960 movie is a masterpiece, “and now we’re going to do our version of it”.

There’s an adaptation of Friedrich Dürrenmatt’s The Visit, reset in upstate New York, destined for the National, and a likely revival of his first play, A Bright Room Called Day, which the Public Theater may take on. Another Spielberg screenplay, The Kidnapping of Edgardo Mortara, about a Jewish boy caught up in the Italian Risorgimento, is on hold until the right child actor can be found. Spielberg says the kid is out there.

Duty to entertain … Tony Kushner in London last year, for a revival of Angels in America. Photograph: Antonio Olmos/The Observer

Also in the works: another collaboration with Tesori. “It’s about the death of Eugene O’Neill because, you know, I have such an instinct for commercial success,” Kushner jokes. It began as A Blizzard on Marblehead Neck, a one-act opera, which premiered in 2011 at the Glimmerglass opera festival in upstate New York, but there are plans to transform it into a two-act opera and then maybe a musical. “I just need to finish it,” he says. “O’Neill doesn’t die in the first bit; now I have to kill him off.”

And a year or so ago, Kushner, who famously wrote Donald Trump’s mentor, Roy Cohn, into Angels in America, told a reporter that he was working on a play about Trump.

Donald Trump has no working intellect as far as we can tell, he's a series of noise effects. Rage and appetite and terror and bullying, that's all he's ever been

That’s still mostly true, but the play, he says, “kept evolving and evolving and evolving and now it’s become something very different”. He’s not even sure if he should call it a Trump play any more and he thinks it may be too soon to write it at all. His plays have a weird habit of prescience – there’s a line in Homebody/Kabul, written in 1998, about the Taliban “coming to New York” and in Caroline activists topple a confederate statue – but Kushner says that he usually needs the luxury of hindsight. “I’m not going to rush to stake my claim in the Trump interpretation sweepstakes,” he says.

And he makes it very clear that Trump will not appear. Why? Because he’s impossible to write about in an interesting way. “The worst thing about Donald Trump is how profoundly boring he is. This man has no working intellect as far as we can tell, he’s a series of noise effects. Rage and appetite and terror and bullying, that’s all he is, that’s all he’s ever been and that’s all he’ll ever be. It’s soul-killing to wake up every day and he’s the president of the United States,” he says. “It’s this big, open sinkhole instead of a soul. This creature, it’s like an id fragment.”

Besides, he says, “The worst thing you can do to Trump is to not talk about him”.