This weekend, Brie Larson is going to win her first Best Actress Oscar for Room. Trust me. It will be a well-deserved honour after her complex, emotional performance in the film, and a long-overdue reward for one of the most promising acting careers around. Larson was arguably even better in the tiny indie film Short Term 12 back in 2013 and she’s also impressed in Trainwreck and 21 Jump Street over the years. Despite being currently known for her sensitive, naturalistic style of acting, one of Larson’s best performances was as the scene-stealing queen bitch Envy Adams in 2010’s Scott Pilgrim vs. The World.

Envy Adams is a near-mythical presence in Scott Pilgrim before we finally meet her in person. We learn that she’s Scott’s (Michael Cera) ex-girlfriend, and his mood towards her tells us all we need to know. He seems traumatised any time she’s mentioned, with his sister, played by Anna Kendrick, forced to refer to her as “She Who Must Not Be Named”. Just who is this woman that’s so terrifying she’s been given a Voldemort-like pseudonym?

We first catch glimpses of her in adverts for her band, The Clash at Demonhead, plastered around Scott’s native Toronto. She wears a striking white leather jacket, with platinum blonde hair and blood-red lipstick. The effect is formidable and it makes two things clear. Envy Adams is the very definition of a rockstar and someone not to be fucked with.

It’s a very different look from her more practical outfits in the likes of Room and Short Term 12. So different in fact, that Larson is initially unrecognisable in her new eye-catching, glamorous appearance. Even more transformative is her demeanour, full of emotionless stares and unimpressed glares. Larson’s recent work has shown she’s more than capable of playing tough characters, but no one who’s quite as much of a buzzkill as Envy. The joy of the role is that playing it so stone-faced only makes it all the more entertaining.

Sitting backstage with her current boyfriend Todd (Brandon Routh), Scott and Ramona (Mary Elizabeth Winstead) she manages to deliver some of the film’s most cutting put-downs. It’s already an awkward situation – Envy and Scott used to date, as did Ramona and Todd – and Envy has no intention of making it any better. “Ramona, I like your outfit. Affordable,” she sneers across the room. Minutes later she’s proudly announcing that, “being vegan just makes you better than most people”. When Brie says it, you can’t help but agree.

A big part of Envy’s terrifying image for Scott is her sex appeal. Although at one point in the film he’s technically dating Knives (Ellen Wong) and Ramona at the same time, he’s hardly a Casanova. He gets very awkward in any sexual or romantic situation, and the fact that Envy is his ex only makes her flirtatious behaviour that much worse. When she invites Scott to her band’s show with the words, “You should So. Totally. Come” -the final part delivered in a breathy whisper – the implication is clear. At that moment it feels like Envy is about to eat Scott alive.

Larson continues that mood when she struts on stage a few scenes later in one of the greatest entrances you’ll ever see. As the elastic bass of her band’s song ‘Black Sheep’ throbs out of Todd’s (Brandon Routh) guitar, she walks onstage and removes her jacket with repeated cries of “Oh yeah!” that can only be described as orgasmic. She then launches into the song’s first verse, bristling with attitude and backed by a stomping rhythm section. Director Edgar Wright and editors Jonathan Amos and Paul Machliss contribute to the ferocity with dynamic split-screen cutting that makes every word feel like a hammer blow. Larson has experience as a singer and it shows in her performance. She completely owns the stage and leaves you in no doubt that Envy is a real rockstar and someone Scott has every reason to be afraid of. Who else is cool enough to have Superman on bass?

This role requires a very different approach to Larson’s recent work in the more naturalistic Room and Short Term 12. The world of Scott Pilgrim is heightened, theatrical and cartoon-y. Larson slots into that different style of acting seamlessly and plays her role as the villainous nightmare ex with real relish. Nevertheless, despite the ice queen persona Larson builds up around Envy, she’s still able to bring real humanity to the character. In her final scene, Scott addresses her by her real name and suddenly Envy’s emotionless expression looks less like someone trying to be tough and more like someone who’s lost. “Natalie? No one calls me that anymore,” she whispers. Remarkably, even after 90 minutes of building Envy up as a hellish bitch, Larson manages to make us feel sorry for her. If that’s not the sign of a great performance, then what is?