No one could accuse Renée Zellweger of not giving everything she’s got. Watching her portray Judy Garland in her new film, Judy, it’s as though she dissolves, molecule by molecule, into the role. Directed by the renowned British stage director Rupert Goold and adapted from the Olivier and Tony-nominated play by Peter Quilter, Judy is set in 1969, when Garland, 46, broke, fraught, separated from her children, did a run of shows at the Talk of the Town nightclub in London. Garland is depicted drinking, pill-popping, impetuously getting married, hanging out with gay fans, sometimes wowing audiences, other times getting booed. There’s Zellweger’s physical transformation with contact lenses and prosthetics, but it’s also in her insolent swagger over to the microphone when Garland is too drunk to perform. The slightly wild stare that begs you to look at her, but also implores you to look away. Zellweger even sings in character – live! A climactic hoarse rendition of Over the Rainbow is particularly poignant. With every gulp, twitch and shiver, Zellweger, starring alongside Michael Gambon, Jessie Buckley, and Rufus Sewell, makes it her business to be Judy Garland, and at times it’s almost painful to watch.

It’s a turn that has generated much awards buzz. Variety magazine said: “Zellweger offers an all-singing, all-dancing, all-collapsing performance of a star at her lowest physical and psychological ebb.” At the recent Toronto international film festival, there was such an extended and rapturous standing ovation – one journalist later tweeting that she’d seen nothing like it in 15 years of attending the festival – that Zellweger eventually had to tell the audience to “quit it”, because her tears were “messing up” her makeup. While we Brits are hyper-aware of Zellweger from the Bridget Jones films, of course her resumé is far more varied than that, from her Girl Friday murmuring “You had me at hello” in Jerry Maguire (1996), to Roxie in Chicago (2002), where she proved she could sing and dance, to her spirited farm hand in Cold Mountain (2003), for which she won an Oscar, and, more recently, in What/If on Netflix – where she camped it up as predatory Anne Montgomery (think Barbara Stanwyck via Cruella de Vil). Now, with Judy, Zellweger offers a study in wrecked humanity – it wasn’t long after this film was set that Garland was found dead in her rented mews house in Belgravia following an accidental overdose of barbiturates.

I’m waiting in the London hotel suite where we’re scheduled to meet when Zellweger pops her head around a corner to chirp “Hello!” She’s wearing a fawn-coloured outfit and pointy red shoes (her own ruby slippers?), with her hair partly pulled back. When I tell her that she looks chic, she says: “Oh thanks very much. It’s only by mistake that I look like this today.” There are some “Hollywood trappings” (a PA type hovers in the background), but there’s no starriness about Zellweger – she just plonks down on the sofa, tucking her feet beneath herself. If she is guarded about some questions, it’s understandable. There are some tough subjects: the claim that Harvey Weinstein used her name to sexually pressure other actresses; the world erupting over her “changed” appearance at the Elle Woman in Hollywood awards in 2014. Otherwise, Zellweger is softly spoken, smart, reflective, and brimming with the down-to-earth Texas charm of her upbringing. When I observe that playing Garland is one thing, but singing as her must be a whole new level of horror (perhaps I could have phrased this more delicately), she starts giggling at the word “horror”. “That was really well put… No, no, don’t take it back – you’re 100% right!”

Asked about the differences between playing Roxie and Garland, Zellweger ticks off a list. “Live performance. Live audience. Real person. Person that I care about. Person who’s meant a lot to me and millions, through generations.” Judy is also about the cruelty of the old Hollywood studio system – with scenes showing the young Judy (Darci Shaw) being controlled, nagged about her weight, and started on a regime of uppers and downers that would ruin her for life. Zellweger says that thankfully there are stricter child labour laws these days. As to whether modern female performers are better off generally, she’s uncertain: “Because you see the Amy Winehouse documentary…”

Garland’s daughter, Liza Minnelli, criticised Judy – posting on her Facebook page that she wasn’t sanctioning the film, and had never met nor spoken to Zellweger, writing: “Any reports to the contrary are 100% fiction.” Does Zellweger think that was because Minnelli hadn’t seen Judy, and would find any film about her mother difficult? There’s a pause while Zellweger considers the question. “I couldn’t say, I don’t know,” she says eventually. “I wish she was here so we could chat!” Speaking to her, it’s clear that Zellweger took pains to avoid producing an “impersonation” or a caricature of Garland. “She’s such a legend. But there’s so much to the human being as well. You want her to come across as big as she was, but also show the woman… Her intelligence, determination, tenacity, hope. She never gave that up, no matter the circumstances.”

Zellweger, 50, grew up in Katy, Texas, and studied at the University of Texas in Austin. “I consider Austin home, and I have a lot of extended family there, so I get back a lot,” she says. From early childhood, she wanted to be a writer. “When the Easter bunny was coming, I would always wish for the same thing – I loved a tablet of fresh ruled paper and sharp pencils. There’s nothing better – stories, poetry, the possibilities were endless.” Her mother was Norwegian, and her father Swiss, which made her feel different. “I very much feel my Texan roots, but there’s no denying that I’m a child of European people.” While at university, she “fell” into acting roles. “Rather than continue on to get my journalism degree, I finished up quickly with my lit degree, just to see where [acting] would go,” she says. “I figured I could always go back and get a master’s, and finally submit to the Daily Texan!”

Renee Zellweger as Judy Garland in Judy. Photograph: David Hindley/Allstar/BBC Films

There seems a certain piquancy to Zellweger aiming to become a journalist when she must have often felt like media prey. Then again, Zellweger tells me that she usually has no problems wandering around incognito. She famously stepped back from screen-acting for six years in 2010, and thinks that helps her in the street. “Taking that time away, when you’re not in people’s consciousness, they think: ‘Oh yeah, she looks like somebody, maybe, but, um, who? I don’t know…’” Zellweger gives her distinctive throaty chuckle: “I sneak around really, really well.” She’s also good at being private, despite high profile relationships, including Jim Carrey, Jack White, a short marriage to Kenny Chesney, Bradley Cooper and, latterly, musician Doyle Bramhall II (just before I meet her, there are rumours that they’d split). Is she in a relationship? Zellweger laughs airily: “I’m in a million relationships!” She says she doesn’t find it difficult to have a personal life: “It’s not difficult at all. You just don’t talk about it, you don’t post it… I know it’s so boring when you’re trying to talk to an actress and she’s being all cryptic and whatever. But I like to keep it…” – she mimes a zipping action – “I just do.” It’s also about work: “It isn’t easy to be an actor who disappears when you have no mystery.” It’s a distraction when there’s too much of a public persona? “Yeah, I think a little mystery never hurt a girl.”

I’d been wondering whether it’s partly about how roles such as Bridget Jones utilise Zellweger’s everywoman quality (warmth, approachability) – when people feel they know someone, maybe there’s also an element of feeling they own them? Whatever’s going on, Zellweger says it doesn’t “exist” for her, because she doesn’t read, watch, or in any way seek out what people are saying about her. She just ignores it? “Yeah, because there’s no upside to not ignoring it. I wouldn’t learn anything from it. It wouldn’t be helpful. What would I do? Explain myself? Or try to somehow mitigate a rumour that someone has started? It seems like a lot of wasted energy to me. I could just be with my dogs, or FaceTime my nephew, or work on writing something. That’s a lot of energy that you would just sort of haemorrhage. All day. On things that you can’t control anyway. And why would you want to?”

One thing Zellweger did respond to was the Elle Woman in Hollywood awards furore, which could only be described as a global pile-on – for a while, it seemed like the entire world was accusing her of having plastic surgery. As it happens, the day I meet her, Zellweger looks exactly like herself, but that’s not the point – even if those 2014 photos did look unlike her, it doesn’t justify what she endured. Zellweger ended up penning an article for the Huffington Post, titled “We can do better”. She wrote: “Not that it’s anyone’s business, but I did not make a decision to alter my face and have surgery on my eyes” – but the article was about much more than that, debating everything from intrusion to women’s worth. To make sure I’m not putting words into her mouth, I double-check with Zellweger that she wrote it (she did). It’s not only an impressive, erudite read (the inner writer is on full beam), it’s also brave. “It just felt necessary,” she says. “I don’t know – was it really brave – if there’s no consequence?” She didn’t know that when she wrote it. “Yeah, I did, I did,” she says quietly. “What’s the worst thing that’s going to happen? What else could be said? How else could I have been made fun of? Or whatever happened… I know what that felt like, and I survived it, so what else?”

Zellweger was also dragged into #MeToo. As part of the ongoing class-action lawsuit, actor Melissa Sagemiller claimed that Harvey Weinstein told her that Zellweger and Charlize Theron had given “sexual favours” for roles. There was a robust response from the Zellweger camp: “If Harvey said that, he’s full of shit.” (Though Zellweger says she didn’t personally put out a statement about it.) Regarding what Weinstein is alleged to have said about her, Zellweger says: “I did hear about that, yes.” Generally speaking, did Zellweger have any bad experiences in the industry? “Never directly in that way. But, you know, there were certain behaviours that I would tolerate because I had a job to do, and I was more interested in getting through the day than in correcting someone, certainly. They weren’t offences of the variety that you’re referencing. But there were certain languages and liberties that certain people would take. But they would take them from anyone, in my estimation, not just me as a woman. But I’m sure there were countless instances that I was not aware of – in terms of deals that were made and conversations that were had, that were derogatory, that I was not privy to. Sure. Of course. I’d be naive to think that I was immune.”

Zellweger with Colin Firth and Patrick Dempsey in Bridget Jones’s Baby. Photograph: Giles Keyte/AP

Zellweger identifies as a feminist, and admires the women who came forward: “It gives you strength in numbers, doesn’t it? It enables you to, in some way, begin to heal.” She seems cautiously optimistic about changes in Hollywood: “I think it’s also right at this very delicate moment where there’s fear and so everybody is hyper-aware of the decisions that they make and what they say… It will be really interesting to see where it settles. But change is inevitable because there’s this new generation of women who are coming of age, and producing things, in every industry, not just Hollywood. They’re not waiting for permission because they don’t need to – and it never occurred to them that they needed to.”

Considering the relentless nature of Hollywood, Zellweger’s six-year career break was a gamble. Was it a work-sabbatical, a fame-sabbatical, or a combination of both? “I needed to take care of myself,” she says. “It was more about being bored with myself, bored with my performances, and drawing on the same experiences, it just felt like regurgitated life, and I needed to collect some life experiences because the well was a bit dry. And I hadn’t grown in any way for years… It’s just redundancy. Because while the experiences were once-in-a-lifetime, dream-come-true blessings, there’s not really room for anything else. And I needed to learn something else. I needed to grow as a person. To grow up a little bit. And I needed to physically take care of myself… But I didn’t stop working, I still worked, just in a different capacity. I can’t be idle.” She has a work ethic? “Oh yes!”

The interview is winding to a close. Does Zellweger have any reaction to the US political climate? “Every day!” Around the time we meet, President Trump has told four congresswomen to “go back” to the countries they originally came from (three of the four women were in fact born in America). Does this kind of thing concern her? “Every day,” repeats Zellweger. “Of course… It’s very upsetting. And it’s upsetting that it’s not as upsetting to as many folks as it should be.”

Zellweger as Judy Garland. Photograph: David Hindley/Allstar/BBC Films

Zellweger isn’t able to confirm her forthcoming projects, not even if there’s definitely going to be another Bridget Jones film: “I’m not being coy, I promise. I’m always the last to know… I do know that Helen [Fielding] has written a book, so maybe.”

Zellweger once spoke about suffering from impostor syndrome – could that just be a human thing? “Yeah, I’m pretty sure that’s a human thing. For most people… There are brain surgeons and whatnot who have to have a different kind of certitude about their skill.” Might it affect women more? “That we don’t feel entitled to whatever it is that we’ve worked for? That’s possibly true.” Stage fright ate Garland alive, and Zellweger tells me that she suffers from that too. Is that the negative side of creativity, the darkness of it? “The self-doubt? I suppose so. But it’s also a complicated mix of things when there’s an audience involved, when there’s…” –she adopts a melodramatic voice – “public humiliation looming.” She corrects herself, with an arch smile. “Potential public humiliation looming.” She shrugs. “You know, you just want to protect yourself.”

Zellweger has the impostor syndrome under control now: “Not so much that I’d ever take anything for granted – that I’d ever sleep while I was doing something, lest I miss something,” she says wryly. “But it manifests a bit differently now. I can shut ‘her’ up a little more quickly.” Zellweger has won Oscars, Golden Globes, Baftas and more – it must get to the point of “Hey, I can do this!” Zellweger smiles, shifts in her seat. “I don’t really look it that way, I don’t judge it… I just look at it as – that was that moment, and now this is this moment… It’s kind of like education, I suppose – it stays with you. It doesn’t mean you should stop learning.”

Judy is in UK cinemas from 2 October