Beanie Feldstein is eating ice-cream in a red dress that is as bright as strawberry sauce. “This is one of the coolest things I’ve ever worn?” she says, her inflection rising at the end, as it often does, lending even her most confident assertions an equivocal note. The 25-year-old star of Lady Bird, Bad Neighbours 2: Sorority Rising and the new film Booksmart is giggly and amiable. Within seconds of me spotting that her ice-cream is vegan (“I’m allergic to dairy?”), she is recommending vegan bakeries in hipster enclaves of the UK capital that a visiting Californian might not be expected to know about. “London is like my second home,” she trills. A glance at her Instagram feed proves that she is certainly well-acquainted with the area around the Palace theatre, where Harry Potter and the Cursed Child is playing.

“I’m such a Harry Potter nut. My friends in college would be partying, and I’d be, like: ‘Uh, I’m gonna go home and read some Harry Potter.’”

I remind her that her brother Jonah Hill, 10 years her senior, came up with a decent Harry Potter gag in the film Funny People: after seeing the latest instalment, he complains that the actors are now so old that it should be called Harold Potter. “Oh, I don’t remember that! I love it.” One of her favourites of his is 22 Jump Street. “He wrote a joke in it for me. When he’s doing the walk of shame across campus in the morning, carrying his shoes the way a girl would carry her heels, and he says: ‘I just wanna get in my bed and watch Friends.’” She lets out a delighted squeal. “Which is literally all I ever say.”

Feldstein is every bit as funny as her brother, and could be on her way to being just as well known. No one who saw Greta Gerwig’s sharp and snappy Lady Bird could have helped feeling protective toward Feldstein as Julie, the title character’s dopey and devoted best friend. Lady Bird’s allegiances fluctuated, but Julie remained steadfast; Feldstein’s reward was to have complete strangers tell her they wanted her to be their best friend, too. “That was my dream scenario because I’m so obsessed with all my best friends and I love being cosy with everyone.” Her words run together in an excitable torrent with no gaps in between.

She has recently been a regular, alongside British comics such as Kayvan Novak, Matt Berry and Natasia Demetriou, in the TV spin-off of the vampire comedy What We Do in the Shadows. And later this year she will be seen with Paddy Considine and Emma Thompson in the film version of Caitlin Moran’s autobiographical novel How to Build a Girl. First, though, comes Booksmart, a high-school comedy that is as finely detailed as Lady Bird – and also marks the directorial debut of another actor, Olivia Wilde – but with a more rambunctious energy, combustible where Lady Bird was clipped. “Olivia pitched the film as Training Day for high school,” she says. “The stakes are so high it feels like war.” Feldstein and Kaitlyn Dever play Molly and Amy, a pair of work-hard, play-soft teachers’ pets who discover too late that they could have had it all – the hedonism as well as the A-grades. On the eve of leaving high school, they ditch their plans to watch the latest Ken Burns documentary in favour of going berserk at last and experiencing, as Molly puts it, “a seminal fun anecdote”. Hilarity and vomiting ensues.

I wonder how Julie from Lady Bird would have got on with Molly from Booksmart. “Julie is such a sweet, kind, giving spirit. Molly would eat her alive. She would just be, like, flick! – and then flick her across the room, even though they somehow have the same face. It was such a different character for me to play because she is so unrelenting in a way I find quite inspirational, while also being sort of prickly. And she is very guarded, which I am not. Maybe I should be. I’m such an open book.”

It’s hard not to adore Molly when she carefully inserts missing apostrophes into the graffiti in the toilet stalls. Would Feldstein have been writing graffiti or correcting it? “Correcting, for sure. I’m such a rule-follower. Although I do feel I’m not totally a Molly. I’m more like George.” He is the fastidious and dapper drama enthusiast, played by Noah Galvin, who throws his own murder mystery parties. At the age of three, Feldstein – born Elizabeth, but nicknamed Beanie by a nanny – had a Funny Girl birthday bash; the theme of her batmitzvah was vintage New York. Although she grew up on the west coast, in LA’s affluent Cheviot Hills neighbourhood (former residents: Lucille Ball, Buster Keaton, Agnes Moorhead), her parents are New Yorkers, who swept her off to Broadway shows whenever they were back east. Her father has been a tour accountant for Guns N’ Roses and business manager for Madonna, while her mother is a costume designer. “We stuck out like sore thumbs in LA. My mom has this thick Long Island accent. Do you know the Yiddish word ‘geshrai’? It means ‘to exclaim loudly’. She’s always geshrai-ing. Everyone was, like: ‘You guys don’t belong.’” I must look alarmed because she rushes to clarify: “I mean, we weren’t tanned, and we didn’t go to the beach.”

Her eldest brother, Jordan, was also in the entertainment industry: he managed Maroon 5, but died in 2017 at the age of 40 from a pulmonary thromboembolism. Feldstein wrote a touching piece a year later about coping with his death – seeing the world now through “grief glasses”. And she turned to the thinkpiece format again in Please Stop Complimenting Me on My Body, in which she expressed discomfort with the approving reactions she received after inadvertently losing weight during an exhausting year-long Broadway run in Hello, Dolly! alongside Bette Midler. Throughout her childhood, she was told she was too big; only in college did she learn to love her body. “After years of pain,” she wrote, “I had finally found such a beautiful peace, one that most people, no matter what size they are, don’t have. And all of those ‘compliments’ took that away from me. After years of finally not feeling judged by myself or others, all of a sudden I felt so seen.”

It is her middle brother, she says, who has taught her to have confidence in her own opinions. “The biggest thing I’ve learned from Jonah is to value my own voice. I’m such an opinionated person in my private life, and he encourages me to be opinionated in my work life, too.” Stories of sibling rivalry and discord are conspicuous by their absence. To celebrate his sister’s appearance in Hello, Dolly!, Hill even had the message “Hello, Beanie!” tattooed on his forearm. “He told me he was going to do it and I just said: ‘Ha-ha.’ Then he came in with it and – oh my God, it takes up most of his arm!” Will she reciprocate? “I totally would have done, except I’m allergic to everything. But I have a sticker of him on my luggage. Does that count? Me at baggage claim: ‘Um, that’s my bag with my brother’s face on it, can I just, um – excuse me – pardon me.’”

While he was directing his first film, Mid90s, she was in the UK’s Midlands shooting How to Build a Girl. She had the advantage of never having seen or heard Moran when she read the script, leaving her liberated from doing any sort of impersonation of her as Johanna, a misfit on her way to becoming a music journalist. “That was such a benefit. And Johanna is only a fictionalised version of Caitlin. The story is true-ish, that’s what we’re saying.” Nor was she familiar with Wolverhampton. “None of my London friends even knew where it was. But now I’ve got such love for the people there. They were so welcoming. I worked for three weeks in Shop in the Square, this feminist utopia in the middle of Wolverhampton. Six hours a day, I was ringing things up, serving customers, working on my accent. And I pulled a pint at the local pub! I felt like the mayor of Wolverhampton.” Now she is pining for British colloquialisms. “I love that word ‘knackered’. I really miss it. If it’s heard among future generations of Americans, you’ll know it’s because of me.”

Booksmart is released in the UK on Monday. How to Build a Girl opens later this year