Dutta told The Indian Express newspaper he noticed the 33-year-old Bollywood star cried when she heard sad stories.
This made her the perfect choice to play courtesan Ameeran, or Umrao Jaan Ada, kidnapped from her home and sold to a brothel in the old Indian kingdom of Oudh, he said.
"Ash has a very soft heart. Even if she hears a rank stranger's sad story, tears well up in her eyes," said Dutta. "I've observed that aspect of her personality and felt that she could react beautifully to the tragedy of Umrao Jaan."