Complex sentence: Unlike Chopra (who met Mary Kom before the film), Kumar did not met Onler Kom before the film, working mainly from videos provided by Kom to the makers, and helping the actor to understand the nuances of his character.
Split into simpler sentences: Unlike Chopra (who met Mary Kom before the film), Kumar did not met Onler Kom before the film. But, he saw videos, provided by Kom to the makers which proved a great help for the actor to get the nuances of his character.