Duffy was cast in 'Legend' on the back of her hit single 'Mercy'.
The 31-year-old Welsh star appears as Reggie Kray's favourite singer, Timi Yuro, in the new movie because of their striking similarities and director Brian Helgeland has revealed how her casting came to fruition.
He said: ''There was a guy who was in 'The Firm' who went to prison with the Krays named Chris Lambrianou and I asked him who Reggie's favourite singer was because I knew music was going to be important.
''He said to me Timi Yuro was his favourite singer. Chris told me how he would book her in all the clubs because he liked her so much.''
The 54-year-old director said having Duffy play the part seemed like a ''fun'' idea.
He told NME: ''I thought that would be a great thread through the movie, to have that live performance from Timi, and I had seen Duffy's video for 'Mercy' - which was very classy, with her up on stage as a nightclub singer.
''So one of the casting directors just got a hold of her. She thought it sounded like a fun thing to do.''
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