Deepika Padukone was offered the lead role during her modeling days before she took up Om Shanti Om (2007), but at the time she wasn't sure about a career in acting. Imtiaz Ali promised her that if she ever decided on an acting career, he would have this role ready for her. Subsequently, she joined acting and Imtiaz Ali signed her.
Giselli Monteiro auditioned for the role of Pae (Saif Ali Khan's non-Indian girlfriend). At this point, Imtiaz Ali had searched all over India for the girl to play Harleen--unsuccessfully. After seeing her screen test, Imtiaz Ali's wife suggested Giselli for the role of Harleen.
The film was originally titled "Elastic."
After getting unanimous acclaim for Quando nos Encontrarmos (2007), Kareena Kapoor really wanted the female lead in the film, especially after her boyfriend Saif Ali Khan was given the male lead. However, upon seeing Deepika Padukone's performance, she said that Deepika was perfect for the role.
During Saif and Rishi Kapoor's meet-ups, a very subtle but important thing the filmmakers did was to give Rishi Kapoor his very own Old Black Tea Glass and give Saif a modern cup/mug; even both of their black teas were slightly different. Nothing was mentioned about it but it adds to their characters. was bit different.