The actor appears opposite Tamil star Dhanush in the romance directed by Tanu Weds Manu maker Anand L Rai.
A source told Bollywood Hungama: "Aanandji was very clear that he wanted NO makeup at all. Sonam had her own interpretation of what 'no make-up' meant.
"There was much back-and-forth of opinion on this between them, since Sonam is a very fashion-conscious actress. She had never appeared without make-up on screen and to do so, she needed to build a new kind of self-confidence. To her credit, she did finally relent."
Designer Roberto Cavalli has described Kapoor as one of the most beautiful women in India.
Kapoor admitted recently that critical acclaim has stopped her career from going downhill after a series of flops.
Watch Sonam Kapoor in the trailer for Bhaag Milka Bhaag: