The French actor was unsure about his likeness to the late fashion designer - who founded his eponymous fashion house in 1961 - until Saint Laurent's former lover and business partner Pierre Bergé broke into sobs when he saw him emerge in full costume.
Pierre told Metropolitan magazine: ''When I made my entrance following two hours in make-up, Pierre Bergé started to sob. The guy who knew YSL better than anyone in the world had glimpsed something of his friend and lover.
''It's the one thing that makes me think that we perhaps really managed to capture something.''
The 24-year-old rising star struggled with imitating the Saint Laurent's distinctive voice, so worked extra hard on his elocution to make the couturier come to life on camera.
He said: ''YSL has such an unusual voice and elocution that I didn't want to tackle it in the wrong way. I wanted to work on it as much as I could until I came up with something that at least I was happy with.
''The voice contained all his fragility; however, I also believe that he used his shyness as a weapon his entire life.''
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