The Men in Black star admits he never got the sense he was chatting to the real Leto in between takes on the set of the movie because the Dallas Buyers Club star remained in character as The Joker.

"We worked together for six months and we've never exchanged a word outside of "Action" and "Cut," Smith tells Beats Radio 1. "We've never said, 'Hello'. (I'm) looking forward to meeting him. I've only ever spoken to him as (my character) Deadshot, and him as The Joker. I literally have not met him yet. So, the first time I see him will be, 'Hey, Jared, what's up? How you doing? Nice to meet you'. He was all in on The Joker."

Leto is known for transforming into his film characters - in 2013, the Thirty Seconds to Mars frontman dropped 30 pounds (13.6 kilograms) to play a cross-dressing, HIV-positive drug addict in Dallas Buyers Club alongside Matthew MCConaughey. He subsequently won a Best Supporting Actor Oscar for the role.

He stayed in character as Rayon in that film too.

"It (role) demanded my full attention," Leto told the New York Times in 2013. "I couldn't imagine every time the director yelled 'Action' that I would recall all of the aspects, the physical and emotional characteristics of the character, at a moment's notice."