"I was skateboarding," the 21-year-old explains, a little sheepishly.
He hadn't skateboarded in years but had taken it up in preparation for a role. That's the story he's sticking to, anyway.
That said, Lerman doesn't strike you as the type of bloke who spends his spare time on a board. He may have only just turned 21 but he's something of an old soul, or at least a little world-weary - as only you can when you've spent the majority of your life in movies.
Born in Beverley Hills, Lerman was apparently two years old when he declared to his mum (who's also his manager) that he wanted to be an actor.
By the age of four he had an agent, and four years after that he was cast in first big screen role, as the youngest son of Mel Gibson's character in The Patriot. He must have made an impression as, that very same year, he played the young version of Gibson's character in What Women Want. A small part in Riding In Cars With Boys followed, as did a couple of TV movies and then, in 2004, he was cast in the US TV series Jack & Bobby.
In recent years, he's starred in movies as diverse as western 3:10 To Yuma, period romp The Three Musketeers and indie coming-of-age drama The Perks of Being A Wallflower, while a Second World War drama with Brad Pitt's in the pipeline.
First though, is Percy Jackson: Sea of Monsters, the sequel to 2010's Percy Jackson & The Lightening Thief, in which Lerman returns as the titular hero and demigod son of Poseidon (the Greek god of the sea).
He may have saved the world in the first film but poor Percy's now suffering a crisis of confidence.
"He's very insecure and self-conscious. He has a lot of doubt in himself, and has decided he's just going to be a regular old demigod," explains Lerman.
He doesn't have much time to brood though, before the magical borders of Camp Half-Blood begin to fall and a horde of mythical monsters threaten to destroy the haven for the children of the gods. Once again, it's down to Percy to save the day - this time by finding the fabled Golden Fleece.
The movies are based on the novels by Rick Riordan, who taught Greek mythology and came up with the idea for the first book after creating bedtime stories for his son.
"Harry Potter is an obvious comparison," says Lerman. "They have similar, specific themes that are universal for the age group that reads these books, and readers respond to that."
In the film, Percy and his pals embark on a journey that takes them from Washington DC to Florida, where they set sail on an odyssey into the unchartered Sea Of Monsters. Terrifying monsters aside, his world is further complicated when he discovers he has a half-brother, a Cyclops by the name of Tyson.
"It becomes a story about the importance of brotherhood and friendship, and also discovering your self worth and having confidence in yourself," says Lerman.
Lerman makes no secret of the fact he'd like to become a director.
"Like George Clooney, there's a small handful of guys who've managed to jump from acting to directing," he says. "I'd love to learn how to tell a story, rather than aid someone else's storytelling."
n Percy Jackson: Sea of Monsters (PG). Out now