Idris Elba thought he had been shot in 'American Gangster'

The 'Alien' director told the 'Luther' star to lean his head against the barrel – to draw a reaction from the actor and add realism to the scene.

Ridley told MailOnline: "He's in the cafe and he says 'just stay here'

and he goes out and all his brothers are watching, and he goes down the street, and Idris Elba is standing there saying, 'Hey, what are you going to do, shoot me Frank, go on, shoot me Frank in front of all these people.'

"And he just goes boom and shoots him on the spot."

Ridley revealed that he would never risk an actor's safety on set, especially in the light of Alec Baldwin fatally shooting cinematographer Halyna Hutchins on the set of the Western 'Rust'.

The acclaimed director explained: "What happened was, I said to Idris, 'Listen, when he puts the gun on your head lean on the gun' – because by the way this is a gun with a solid barrel, there is no aperture, I would never risk it – but when you pull the trigger there's a recoil, there's no blank, nothing."

Ridley added: "He pulled the trigger and it goes 'Bang'. Idris thought he'd been shot and dropped to the sidewalk and said, 'I've been shot!'"

Firearms expert Paul Biddiss, an advisor on weaponry for films, who is currently working with Ridley, discussed the 'Gladiator' director's strict safety procedures on the set of 'American Gangster'.

Biddiss said: "He is very hot on safety.

"What he was referring to by a solid barrel is a completely filled-in barrel that no bullet would be able to pass through, so it was a completely deactivated impractical gun as far as firing anything was concerned."

The firearms expert admitted he was puzzled by the use of the term 'cold gun' on the set of 'Rust'.

He said: "I've never heard that expression cold gun. George Clooney, who I was with on my first film, says he hasn't heard it either."