Kashyap originally wanted to cast Konkona, but due to her pregnancy in 2010 and early 2011, she wasn't available for the shooting.
The filmmaker's second option was Nandita Sen, but she too couldn't commit time for the movie, says the source.
Finally, Reema was approached and she signed the film soon enough.
'Gangs Of Wasseypur' is a two-part film, dealing with the coal mafia. It will release June 22.