Rows of students line up outside the exam hall. They hold pencil cases and clear plastic folders. Some are chatting; others gaze nervously into the middle distance. Then the line begins to move and they file into the hall past watching invigilators.
"And ... cut there!"
A make-up woman pounces on milling students, combing windswept hair and spraying it back into place. Someone dashes down the corridor, clutching a sheaf of continuity photos.
This is the set of X Plus Y, a new film about life, love and the International Mathematical Olympiad.
The film, which will be released next year, follows teenage school student Nathan, who struggles to build relationships with people, including his own mother, but seeks comfort in his prodigious ability with numbers.
Mentored by unconventional teacher Mr Humphreys, Nathan earns a place representing Britain in the International Mathematical Olympiad. But, when the team travels to Taiwan, Nathan must deal with unfamiliar surroundings and experiences, including his irrational, illogical feelings for his Chinese opponent Zhang Mei.
And so, on a rainy day in mid August, more than 100 teenage extras are lining up in the quadrangles of St John's College, Cambridge, trying to act as if they are about to sit an important exam. ("We did have more extras, but today is A-level results day, so that makes it a bit tricky," says Laura Hastings-Smith, one of the film's producers.)
At the entrance to the college, International Mathematical Olympiad flags flutter in the breeze. During a break from filming, Rafe Spall, the actor playing Mr Humphreys, lounges on the college lawn. His hair is pinned back with hairdressers' clips; a gnarled wooden staff is at his side. Wild hair and a walking stick are, apparently, the accoutrements of the eccentric mathematics teacher.
"I have absolutely no affinity with maths whatsoever," says Spall (pictured left). "I'm not in any way mathematically talented. We've got a maths teacher on set and he showed me Euler's totient. And I can write out Euler's totient with great deftness and speed, but I've no idea what it means. Euler was a famous mathematician, but I've no idea what his totient is."
Spall previously played the unnamed author in Life of Pi, the film of Yann Martel's novel, and starred as aspiring comedian Ian in One Day. "It's all smoke and mirrors," he says. "That's what acting is. You pretend that you know about stuff. You're able to speak about a lot of stuff, but should the person you're speaking to scratch beneath the surface, you'll realise that the reservoir is very shallow."
The job of teaching mathematics - or at least a convincing impression of it - to Spall and the rest of the cast falls to Lee Zhao, a mathematics PhD student at the University of Cambridge. "Some of the actors had this idea that, if you were good at maths, you were some kind of genius," he says. "But the more I explained, the more they realised that it's not as mysterious as it seemed."
Unlike Spall, some of the actors genuinely wanted to understand more about the mathematics that they would be discussing during filming. And, Zhao says, he was entirely happy to dispel his own mystique.
"I'm not going to be the Wizard of Oz here and pretend I'm almighty and great," he says. "It's not a magical thing that you can or can't do.
"It's not too dissimilar from anything else you excel at. Some people might be better than others, but you still have to put the hours in, to practise. Even if you find maths hard, if you put the hours in you can learn how to do it."
Many of the cast members admit that they simply cannot imagine what it would be like to grow up viewing the world mathematically. "I love the way that the world of maths unlocks the character of Nathan," says Sally Hawkins, who plays Nathan's mother. "Maths - it sort of opens his eyes. He can use his brain in a way that others can't. It's a rare, special thing."
But Hawkins, who appeared in the 2012 film adaptation of Great Expectations and is in the Godzilla remake being released next year, says that she was never particularly good at the subject. "At GCSE, I had an amazing teacher, and I went from 'Where were we?' to 'I can do this'. But (it was) simple sums."
Zhao's role is twofold. In addition to explaining the mathematics, he was a member of the UK team during the 2006 Olympiad and is able to tell cast members what it felt like to enter the competition.
"Lots of people are aware that there are youth things for sports, but young people can excel in many different ways," he says. "It's quite nice to see a film about brilliant mathematicians. I hope people see this and take away that, apart from the maths, you're just a run-of-the-mill teenager."
Hastings-Smith echoes this. "I guess there's a kind of geek chic, isn't there?" she says, looking out at punters on the River Cam, several of whom are wearing Olympiad T-shirts. It is unclear whether they are actors or would-be undergraduates.
"There's something really intriguing about a group of people who are so clever at something. It's a kind of intriguing mystery, really. But they're people who are often a little bit different, as well, and they have their own charms," Hastings-Smith adds. "Cinema is about entering other worlds, isn't it? It's about having different worlds opened up to you. And this is part of that."