Joaquin Phoenix ''started to go mad'' while preparing for his lead role in 'Joker'.

The 44-year-old actor shed 52lbs to portray Batman's most famous foe in the upcoming DC Comics origins story and admitted his rapid weightloss helped him get into character.

He explained: ''It turns out that affects your psychology. You start to go mad.''

While Joaquin lost the weight quickly, one aspect of the role that took him some time to grasp was his character's distinctive laugh and he even made director Todd Phillips ''audition'' it to ensure he'd got the sound right.

He said: ''[It needed to be] something that's almost painful. I think for Joker it's a part of him that wants to emerge. I think we all kind of assume what a Joker laugh is and it felt like a new, fresh way of looking at it.

''I didn't think that I could do it. I kind of practiced alone but I asked Todd to come over to audition my laugh. I felt like I had to be able to do it on the spot and in front of somebody else. It was really uncomfortable. It took me a long time.''

The Joker has previously been played by the likes of Jack Nicholson, the late Heath Ledger, and Jared Leto, but Joaquin deliberately stayed away from looking at other incarnations of the role.

Speaking at the Venice Film Festival, where the movie had its world premiere, he said: ''For me the attraction to make this film was we were going to approach it in our own way. I didn't refer to any past iterations of [the Joker]. It just felt like our own creations, which was really important to me and the key to it.''