Sound

Sound in Bollywood films is rarely recorded on location (otherwise known as sync sound). Therefore, the sound is usually created (or recreated) entirely in the studio, with the actors reciting their lines as their images appear on-screen in the studio in the process known as "looping in the sound" or ADR—with the foley and sound effects added later. This creates several problems, since the sound in these films usually occurs a frame or two earlier or later than the mouth movements or gestures. The actors have to act twice—once on-location, once in the studio—and the emotional level on set is often very difficult to recreate. Commercial Indian films—not just the Hindi-language variety—are known for their lack of ambient sound, so there is a strange silence underlying everything instead of the proper sound to create some sort of depth.
The ubiquity of ADR in Bollywood cinema became prevalent in the early 1960s with the arrival of the Arriflex 3 camera, which required a blimp (cover) in order to shield the sound of the camera, for which it was notorious, from on-location filming. Commercial Indian filmmakers, known for their speed, never bothered to blimp the camera, and its excessive noise required that everything had to be recreated in the studio. Eventually, this became the standard for Indian films.
The trend was bucked in 2001, after a 30-year hiatus of synchronized sound, with the film Lagaan, in which producer-star Aamir Khan insisted that the sound be done on location. This opened up a heated debate on the use and economic feasibility of on-location sound, and several Bollywood films have employed on-location sound since then.