Forest Whitaker found playing Cecil Gaines in 'The Butler' the most ''difficult'' of his career.

The Oscar-winning actor admits playing the film's central protagonist in Lee Daniel's movie was intimating because of the challenging techniques he needed to master, but he's glad he was chosen for the role because it pushed him outside his comfort zone.

Speaking to The Huffington Post, he said: ''I was really nervous that I wasn't going to be able to accomplish this. For me, this is the most specific work I've ever done, and possibly one of the most difficult roles I've ever played. Part of that is because of the silence.''

He added: ''But honestly, I was grateful, because at that point I needed a challenge like this for myself as an artist. I needed to be afraid. Being afraid is good. I knew I was going into new territory. I thought maybe I'll expand myself.''

Meanwhile, the 'Last King of Scotland' star was previously in negotiations to play Martin Luther King in Paul Greengrass' biopic 'Memphis', which will follow Dr. King's final days as he struggles to balance his personal life and the civil rights movement, as well as his eventual assassination at the hands of James Earl Ray in April 1968.

Additionally, it's thought the biopic will focus on the events leading up to his final protest in Memphis, Tennessee and also the aftermath of his death as the FBI mounts a manhunt to find his killer.