Kareena Kapoor was the first choice of director Imtiaz Ali's Rockstar. However, as Ranbir Kapoor had already been finalised for the male lead, they decided against choosing Kareena, who happens to be Ranbir's sister.
Director Imtiaz Ali, who loved Kareena Kapoor's performance in Jab We Met wanted to cast her in Rockstar. However, the script in Rockstar required some romantic scenes between the romantic leads. As Ranbir and Kareena were siblings, this would have been a problem. Moreover, the filmmakers could not imagine replacing Ranbir with any other actor as the character that Ranbir Kapoor plays in Rockstar was tailor made for him. They then made a joint decision to let go of their plan to cast Kareena in Rockstar.
In an interview with a leading newspaper, Ranbir Kapoor admitted that Imtiaz was keen to work with Kareena but decided against doing so as she was his (Ranbir"s) sister. Ranbir Kapoor had also said that he wanted to work with Kareena in the future, provided the roles were not romantic ones!