There is a moment in the trailer for The Perks of Being a Wallflower when it's fair to say many people may do a double take.
Emma Watson, synonymous with the studious, faintly highly strung Hermione Granger in the Harry Potter movies, is heavily made up, dancing in burlesque style and imploring her co-star to be a "psycho". Her accent is perfect as usual - perfectly American. The Hogwarts uniform has been replaced by a high school baseball jacket. At the Toronto International Film Festival last week, she hoped that the film would allow people to see "other sides of me that perhaps they haven't seen yet." You can say that again.
It's an inspired piece of casting - both for Watson and the film's producers. The latter have form in the young adult field - Mr Mudd Productions was responsible for the Oscar-winning adolescent drama, Juno (2007). But The Perks of Being a Wallflower has, in recent years, smacked of a vanity project: the book's author, Stephen Chbosky, himself began adapting the story in 2005, set in 1990s Pittsburgh, and ended up in the director's chair. But it did feel like the film could be yet another whimsical - and slightly annoying - tale of American youth, to be filed alongside 2004's Napoleon Dynamite or Garden State.
And then along came Emma Watson.
The success of the Harry Potter series has naturally given Watson a sense of trepidation about her next move. But her transformation from teenager into twentysomething has been strikingly mature. Modelling for Burberry and Lancôme and working with the fair trade brand, People Tree, meant Watson was often spotted on the fashion circuit, but she still went to university (for a while), maintaining a sense of calm instead of taking on the first movie on offer.
Of course, Watson's Potter earnings have meant she's in the happy position of accepting roles that interest her, rather than those that pay the most. She also knows that she has a ready-made audience who have grown up with her and will always be fascinated to see what she might do next.
After all, there were relatively low expectations earlier this year for Daniel Radcliffe's first major post-Potter movie role in the spooky thriller The Woman in Black. Yet it not only became the biggest US opening for a Hammer film in the company's history, but is now the highest-grossing British horror film in more than 20 years. Radcliffe clearly cast a spell on the whole endeavour.
What Chboskywouldn't give for a similar reaction to The Perks of Being a Wallflower. Anyone who hasn't read the book or seen Watson's name on the posters may well assume, from the uptight character in the Potter series, that she is the titular wallflower. But the story actually follows the journey of Charlie (Logan Lerman), a high-school loner. It's not a plot spoiler to suggest that his stepsister, Sam (Watson), and his effervescent friend, Patrick (Ezra Miller), help him come out of his shell, but not before some dark secrets are revealed. Oh, and Watson does a routine from the raunchy Rocky Horror Picture Show.
Her next projects are fascinating: she's one of a gang of celebrity-fixated LA teenagers who burgle the homes of Hollywood stars in Sophia Coppola's The Bling Ring; there's a brief cameo in Seth Rogen's apocalypse comedy, The End of the World; and she starts work soon on Darren Aronofsky's Biblical epic, Noah.
As for The Perks of Being a Wallflower, early reviews have been decent, if not effusive. But nearly all of them note Watson's performance: confident, natural and not afraid to explore the darker elements of life. Being the best part of an otherwise forgettable, if perfectly watchable, film is no bad thing: leaving Hogwarts behind in such fine style would make Dumbledore very proud indeed.