Jesse Eisenberg admitted his new film, The Double, felt like double the work as he plays both the lead characters.
Richard Ayoade wrote and directed the comedy, inspired by Fyodor Dostoyevsky's novella about a Russian government clerk who is driven mad by the arrival in his life of a man who is his doppelganger, but whose confident and aggressive personality is the exact opposite of his own.
Speaking at the BFI London Film Festival's gala screening, Eisenberg revealed: "It was two characters, so it was the equivalent of doing two different roles, except that each character needs the other to exist so it's not exactly like doing two separate movies."
Mia Wasikowska plays Eisenberg's love interest in the movie, and the pair have since become a real life couple.
Her character is indifferent to Eisenberg's shy character Simon, but finds herself strangely attracted to his evil doppelganger James.
Eisenberg said: "Mia's great, she is very polite to Simon but she pines for James because of the way he looks.
"They're exactly alike but she says, 'There's something about the way he looks that makes me want to be with him!' So it's very odd."
Ayoade is not taking credit for matchmaking the couple by casting them together.
He insisted: "I just thought they were individually very good. I didn't input their names in match.com!
"They're just great and that's where my input ended as being one of many people who realised how terrific they are."
The cast also includes British actors Paddy Considine, Craig Roberts and Yasmin Paige, who starred in Ayoade's critically acclaimed directorial debut Submarine. His IT Crowd co-star Chris O'Dowd also makes an appearance in the film.
Eisenberg said he found it glamorous to be appearing at the London Film Festival.
He said: "For most of the people who worked on the movie this is like the local festival but for me it's this very foreign European thing. So for me it's like this very exotic thing, but for everybody else they go to Giraffe's and then they come here."
The Double is set to be released in cinemas next year.