Bachchan was originally cast in the lead role of Mahi Arora in Heroine. However, the role subsequently went to Kapoor when the Raavan actress announced she was pregnant with her first child.
Speaking to The Times of India, Kapoor said: "Ash is an icon; she is a beauty. She would have done the film differently. And I have approached the role very differently. I don't think I can make any comparisons."
Charting the rise and fall of a Bollywood star, Kapoor said that she had no apprehensions about playing an actress whose career was on the decline.
She added: "Even my mother and my sister asked me whether I was sure that I wanted to do this role. I just felt the need to do something different, and when I say different I do not mean that it has been skin show. Playing a bold and aggressive heroine was a challenge that I enjoyed."
The 3 Idiots star, who has undertaken an intensive promotional campaign for the film, said that she found the experience of Heroine emotionally draining, describing it as "an obsession".
"In a career spanning a decade, Heroine is one of the films I have felt very close to. I have invested so much in this film emotionally that at the end of it, it has left me drained. In fact Mahi is someone who has stayed with me... and in the coming days I need to exorcise her. The film is such an obsession with me; it almost feels like my first-born," she revealed.
The actress, who celebrates her 32nd birthday with the release of the film, said Heroine's success would be her greatest gift.
"I have given this film my 200%. Now I'm waiting for the verdict. Good, bad, ugly, I want to know what the industry and my fans have to say. It is my birthday and the only gift I want today is for my film to succeed. We actors are greedy people; we want praise and more praise."
Heroine releases nationwide in cinemas today (September 21).