Ben Mendelsohn struggled to keep his part in Rogue One a secret, especially when seeing people in Star Wars T-shirts.

The Australian actor has landed a role in the highly-anticipated spin-off movie, though is keeping mom on his character. During an appearance on Jimmy Kimmel Live! on Thursday night (16Jun16), Ben explained how hard it is to keep the project shrouded in secrecy, especially before his involvement was known about.

“The worst thing is, right, you’ll be walking down the street and you’ll see some kid in a Star Wars T-shirt, and you’re like, ‘I’m in that... whether you know that or not,’” he sighed.

Fans of the franchise have been putting pieces of the puzzle together since the trailer for Rogue One: A Star Wars Story dropped earlier this year (16), showing Felicity Jones and Diego Luna as heroic resistance fighters on a mission to steal plans for the evil Death Star ship, first made famous in 1977's A New Hope.

While Ben, 47, won't reveal any further details, his alter ego is seen wearing a uniform of a Grand Admiral in the Imperial Navy, leading people to suspect he's a part of the evil Empire building the Death Star. But what he did divulge during his appearance on Jimmy's chat show was his own adoration for the sci-fi films.

“I was a Star Wars tragic," he recalled. "I was right at that sweet spot. I was seven or something when the first one came out. I fell in love – the whole box and dice.”

The film is due for release in December (16) following the huge success of The Force Awakens which reunited original cast members Harrison Ford, Carrie Fisher and Mark Hamill.