'Yesterday' star Himesh Patel was stunned by the movie's "poignant" surprise cameo.
The actor plays singer songwriter Jack Malik in a world where The Beatles never existed, which leads to an incredibly emotional moment when he meets a 78-year-old John Lennon, who lived a peaceful life out of the spotlight.
Opening up on the touching scene, he told Digital Spy: "It kind of took my breath away, to be honest. It felt so poignant, of course, and yeah, really beautiful.
"This whole film is kind of a 'what if?' story, and it felt like the tip of that, you know? "Maybe there's a universe in which that's where John is. And then he still turns out to be this wonderful, thoughtful, philosophical man."
The actor playing Lennon in the shock cameo appearance requested - along with Universal - to keep his identity under wraps "out of respect" for the late musician's memory.
The studio said in a statement: "The filmmakers and the actor made an agreement that they would keep the actor's identity a secret out of respect for the life and memory of John Lennon. We respect their wishes and therefore will not be releasing his name."
Meanwhile, director Danny Boyle - who also won't be naming the performer behind the role - recently revealed they didn't hold auditions for the special part.
He explained: "We just talked about this actor and I sent it to him. I knew him, and I knew he was a lifelong Lennon fan. For him, it was a special journey.
"He didn't want to be credited in the cast, in the list, and he didn't want it to be talked about or become a thing or anything like that.
"He just wanted to do it to honour someone that was very important in his imaginative life. He never met him or anything like that, but he played a huge part in his imaginative life."
Although the iconic musician survived in the film's story, he died on December 8, 1980 when he was shot by fan Mark Chapman as he returned to his Manhattan apartment hours after signing an autograph for him.