Christian Bale had a very straightforward method for packing on the kilograms for his next movie.

The usually super-fit actor - who will play former US Vice President Dick Cheney in upcoming biopic Backseat - has been spotted out and about in recent months looking almost unrecognisable after gaining a significant amount of weight for the role.

Christian Bale attends a premiere on September 11, 2017 in Toronto, Canada. Photo / Getty
Christian Bale attends a premiere on September 11, 2017 in Toronto, Canada. Photo / Getty

He kept it simple when he was asked about his dramatic transformation at the Toronto International Film Festival, saying: "I've just been eating a lot of pies."

Bale, 43, is certainly no stranger to drastically changing his body for roles.
In 2000, he shed body fat and gained muscle to play Patrick Bateman in American Psycho.

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However, Bale's most famous physical transformation to date came back in 2004, when he played a man with crippling insomnia in The Machinist.

Christian Bale in tanning bed in a scene from the film 'American Psycho', 2000. Photo / Getty
Christian Bale in tanning bed in a scene from the film 'American Psycho', 2000. Photo / Getty

Bale survived on a diet of apples, tuna and cigarettes for four months prior to filming, losing 28kg and bringing his total body weight down to an emaciated 54kg.

After filming finished, the 6ft tall actor then had just six months to bulk up for his role in Batman Begins.