The scene involves Ranbir playing the guitar and singing to an audience of sex workers. Director Imtiaz Ali thought of shooting at an actual brothel, instead of creating a set and using junior artists.
A recce was conducted and the shoot took place at a brothel with actual sex workers. Sources say that the sex workers were initially reluctant to face the camera, bur both Ranbir and Imtiaz pep talked and the participated in the shoot. For that particular scene, Ranbir played the guitar and sang in his own voice.
Click Here to More Rockstar Movie