"It is a pretty weird concept, but it's a brilliant way of celebrating the band and their music," suggests Himesh Patel, who stars as Jack. "It's also easier than doing just a straight biopic on them, which would be a hell of a challenge."
Having to play and sing The Beatles back-catalogue was, admittedly, a "hell of a challenge" too, for Patel – especially as this is his first lead part, with his only previous success being a recurring role in EastEnders, as Tamwar Masood. Having to then perform in front of thousands at Wembley and in Las Vegas, at CinemaCon, has been yet another challenge.
"I'm not a songwriter - I'm not that level of musician - but I do play piano and guitar, although I really had to improve my guitar playing for the movie," he admits. "But I'm just glad to have regained a little bit of confidence when it comes to playing piano, guitar and singing because they'd kind of fallen by the wayside.
"Playing at Caesar's Palace, at CinemaCon, was another thing entirely; it was one of the more surreal things I've done. But what a privilege. I'm proud of myself for having done that and very glad to be given the opportunity to do so," adds Patel. "It was a really weird moment and I don't really remember much of the doing it, because it was pretty daunting."
Equally daunting for Curtis and director Danny Boyle was finding an actor who could believably sing and perform the 15 Beatles songs they'd licensed, at great expense, from Apple music for Yesterday. But just as they were about to give up, in walked Patel and wowed them with his heartfelt, life-changing audition.
"It was proving very tough to find a lead," acknowledges Curtis. "A lot of people were either karaoke or – worse - stage school versions of The Beatles. Then Himesh walked in and he had this very deadpan manner, which you see in the movie. He just had this amazing effect when he sang. The fact that he was Asian was also interesting, because there haven't been any major Asian pop stars from England, so his chances [of success] were probably even lower than Ed Sheeran's, a ginger pop star!"
Speaking of which, Ed Sheeran makes an appearance, as himself, in Yesterday, when he offers Jack a support slot on his European tour, after hearing him on local radio. Getting Sheeran on board was quite a coup and quite a surprise because he turns in a surprisingly good comedic cameo. How did that happen?
"That was also a bit of luck," admits Curtis. "I know Ed quite well, because he lives near us, in Suffolk, where the movie's set and I bumped into him somewhere years ago and my very over-eager girlfriend got his number and rang him up. He always comes and plays football with my boys and pops over every year, at Christmas."
For Lily James, who stars as Ellie - Jack's best friend, sidekick, manager and unrequited love interest - the downsides of his spiralling fame means she's left behind while he's leaving home, with his sights set on stardom and a jet-setting, celebrity lifestyle.
"I can relate to that feeling of unrequited love because I was always falling in love with boys as a teenager," admits James, blushing. "I kept a diary and every day it seemed like I was writing about yet another boy. So, I could connect to that and to Ellie. I actually really liked her, as a character, because she's strong, she's proactive and she's calling the shots; she's not just going to wait for him. I saw myself in her and I think she's a character people can relate to."
Starring in Yesterday also had another, unexpected resonance for James – it brought back poignant memories of her dad, an avid Beatles fan, who passed away when she was just 19.
"My dad was a musician and he had all these old records that he'd use to make mix tapes of, for the car," she recalls. "The Beatles were always on them so every time we went on a car journey I'd always hear their songs, along with lots of other great bands he was into. Doing this film reminds me of that time."
Hearing James recollect her childhood, listening to The Beatles, and watching Patel's passionate, poignant renditions is a powerful reminder – if anyone had forgotten – of just how great the Beatles were, as does Yesterday, which is a fitting, funny, brilliant and bittersweet overview of how lives can change and be changed by music or, in this case, the absence of it. Luckily though, Yesterday is just a film, because who would want to imagine a world without The Beatles, their music and the indelible imprint it's left upon the world?
LOWDOWN:
Who: Himesh Patel and Lily James
What: Yesterday
When: In cinemas today