The usually immaculately dressed singer – who plays dowdy social worker Miss Weiss alongside Lenny Kravitz and Gabourey Sidib in 'Precious: Based on the Novel Push by Sapphire' - loved stripping off her make-up and quality clothes for the part.
She said: 'A lot of people didn't recognise me, and I was actually happy about that. It wouldn't have worked if it had been any other way.'
But the singer-and-actress – renowned for her diva-like demands – did find it difficult to adapt to being unglamorous.
She added: 'I tried to put a little blusher on and Lee (director Lee Daniels) caught me, and he said, 'No! You're not having anything!'
Mariah, 39, previously revealed her own friends and family barely recognised her in the role because she looked so different.
She said: "It was cool, because a lot of people didn't recognise me - they don't notice me - and that to me, was the great gift that Lee gave me.'