The film is based on Sarabjit Singh, an Indian national who was jailed in Pakistan on the charge of spying.
Aishwarya plays the role of his sister Dalbir Kaur.
Omung says he wanted to cast an actress who could pull off being a twenty-year-old and a sixty-year-old with equal ease.
"There are a very few actors who are in the middle range (in age) and mature enough to play a 22-year-old and a 60-year-old also. Young actors can be aged but it looks fake. I wanted that middle person," Omung told PTI.
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"It just took 15 minutes for Aishwarya to say yes to the film. I know her for a long time, since her Miss India days and we have been friends. But I wanted to go to her with a solid role. She saw the full research I had done, I narrated one scene because I didn't have a script then, and she was on board," he said.
Omung was all praise for Aishwarya's professionalism.
"When she is on set, she won't move from one place. If lighting is required, she will herself stand there...If it is a shot change, for continuity purpose she would retain her position for as long as one hour so that nothing should change in her style," he said.
The film is scheduled to release on May 20.