Colin Farrell mistaken for Colin Firth at his gym

Colin Farrell was once mistaken for fellow actor Colin Firth when he tried to take out a gym membership.

The Fantastic Beasts and Where to Find Them star decided to work on his fitness and walked down to the gym to sign up but was shocked when the receptionist tried to phone through to confirm his membership - as she tried to register him as Colin Firth.

"I tried to sign up for gym once and the woman got on the phone to sort out the membership and she said in a rather loud whisper, 'Colin Firth is here,'" Farrell tells British chat show host Graham Norton, in an episode of the program set to air in the U.K. on Friday (13Oct17).

The Irish actor added that he was especially surprised at the receptionist's confusion as The King's Speech star is several inches taller than him.

Farrell's new film, The Killing of a Sacred Deer, sees him play a cardiovascular surgeon who is accosted by a young boy whose father died during one of his operations.

In order to prepare for the role, the star shadowed Dr. Mario Castillo-Sang, a real-life heart surgeon from Cincinnati, Ohio - an experience he found "disturbing".

"I attended a real heart bypass surgery - it was disturbing and fascinating," the 41-year-old explains. "I turned green and had to step out of the room. Even though the patient was alive and survived, it screamed 'cadaver.'"

The film sees him reteam with The Lobster director Yorgos Lanthimos, who he claims originally intended to make a comedy, but ended up making a darker movie.

"The director is a very particular filmmaker," Farrell adds. "It is very dark but I found it fascinating to morph into what he wanted. I think Yorgos thought he had made an outright comedy. But I found it pretty dark and I was depressed by it."

The Killing of a Sacred Deer, which also stars Nicole Kidman and Alicia Silverstone, is set to be released on 3 November (17).