Aishwarya Rai Bachchan was originally offered the role of Neha, but she refused because she felt that her real-life marriage to Abhishek Bachchan would ruin the whole idea of the movie.
While shooting Kabhi Alvida Naa Kehna (2006) in 2005, Abhishek Bachchan was actually offered the role of Kunal while Saif Ali Khan was meant to play Sam when director Tarun Mansukhani first had the concept for the film. However, Saif declined and was replaced by John Abraham. After seeing the potential in John, Tarun then decided that John would be a better choice to play Kunal, and Abhishek to play Sameer.
Saif Ali Khan was initially offered the role of Kunal, but he rejected it because he felt that he would be sidelined by Abhishek Bachchan and Aishwarya Rai Bachchan(who was originally in talks for the lead role) due to their popular marriage.
The first film in India with a main plot based on gay relationships. Before that, homosexuality was shown only in a comic sense, mostly with double entendres. The first film in India which took homosexuality seriously was Girlfriend (2004), based on lesbian relationships.
Amitabh Bachchan, father of Abhishek Bachchan who plays Sam, also has a multi-starrer named Dostana to his credit, which also starred Shatrughan Sinha alongside Bachchan.