"The physical transformation was utterly transfixing and complete, but his interior transformation was mesmerizing," director Scott Cooper told PEOPLE

By Jodi Guglielmi
Updated September 05, 2015 12:25 PM
Credit: Warner Bros.

Johnny Depp is perhaps one of Hollywood’s most versatile actors. From Edward Scissorhands to Caption Jack Sparrow, Depp, 52, is known for his ability to completely transform himself both mentally and physically for a role. In his latest film, Black Mass, Depp becomes almost unrecognizable to play notorious Boston mob boss James “Whitey” Bulger.

He trades in his dark hair and deep brown eyes for a receding whitish-blonde hairline and piercing blue contacts – but it’s not just his physical transformation that is earning praise from early reviews.

“The physical transformation was utterly transfixing and complete, but his interior transformation was mesmerizing,” director Scott Cooper told PEOPLE of Depp’s performance.

Both Depp and Cooper studied surveillance footage and police audio tapes to capture Bulger’s look and “true animalistic behavior.” By the time Depp arrived to set on the first day of filming, he had taken on the mindset of Bulger, who on the surface was charming and personable, but could turn deadly in seconds.

“For him to completely give himself over to Whitey’s psyche and play not only an extremely broken man and haunted man, but a man who is at turns vicious and cunning was memorizing to watch,” said Cooper.

“I think Johnny gives a full body portrait that I hope will rank among some of his great performances of all time,” he added.

The film spans from 1975 to 1995, tracing Bulger s rise up the criminal ranks to become the king of Boston’s underworld, all while being protected by childhood-friend-turned-FBI-agent John Connolly (Joel Edgerton).

Though Depp’s transformation wasn’t easy to complete, Cooper admits that it wasn’t the most difficult part of making the film.

“The true challenge was telling the right story, the true story,” explained Cooper. “Because the truth can often be elusive.”

Catch Black Mass in theaters now.