Though he expected it might be difficult to be a newcomer joining an established cast from the first film, he says he couldn't believe how much on-set life was actually like a Disney movie.
"I remember my first walking into the production office and they were playing table tennis with each other, and sharing a bucket of ice cream. ... It was a lovely environment," he said.
The only thing he could remember being frustrated by was trying to get the horses to stand still while he delivered his lines.
Barnes grew up in a family of doctors in London, studying theater and music throughout his childhood.
He said the only thing he lied about to get the part was being able to ride a horse, and he recalled phoning his mom afterward and asking if in fact he had ever been riding. He joked that he was relieved when she produced a photo of him on Shetland pony at age 6.
Looking back, he said the movies that inspired him most included "everything from 'The Sting' and 'Some Like It Hot' through to 'The Shawhsank Redemption' and 'Spinal Tap' and 'American History X.'"
Though he had made it onto the cast of the U.K. production of the hit play "The History Boys," when the Narnia casting agent found him, he was still relatively unknown.
His once-forgotten teenage stint with the boy band HyRise is now creeping back onto the horizon amid Barnes' press blitz for Prince Caspian, to his dismay.
"My first job was playing the drums in a West End production of 'Bugsy Malone.' ... I was moonlighting as a jazz drummer when I was 16, singing Stevie Wonder at graduations, and we had this one song and now there's no escape. Everyone has a skeleton and that's mine," Barnes said, still managing a smile.
On track to superstardom, he's eager to downplay his role in HyRise to protect his new image, but he says he'll never be a diva and doesn't even feel entitled to be picky about projects yet.
"I'm not in that place yet," he said. "I would like to work with whoever would like to have me."
With a merchandising juggernaut like Disney now promoting his face, Barnes does seem to still have his feet grounded but admits he was "a little disappointed" when he saw the action figure of himself.
"Based on what I did with my Transformers and He-Mans at that age, I'm probably better off. It would be terrifying seeing a kid chewing on the head of a toy with your likeness," Barnes joked.
"The Chronicles of Narnia: Prince Caspian" is produced by the Walt Disney Company, parent company of ABC.