OMG! Why did Kareena Kapoor show middle finger to Saif?

In the scene Happy Ending, Saif's character is dumped by his girlfriend, played by Kareena and the nawab's begum walks out on him after cheekily showing him the finger. Although the scene was always a part of the script, director Krishna DK says that casting Kareena in the cameo was a spontaneous decision.