It is common practice in film recording sessions for conductors to lead the players by use of a "click track" that precisely, to the frame, dictates how long each musical passage should run, to keep the score in sync with the action on screen.
At the "E.T." recording sessions in 1982 (pictured), Williams performed the film's closing passages (which vary frequently in tempo) free from the film, and Spielberg re-edited the ending to match the musical performance - something highly unusual, and to the film's dramatic benefit.
Spielberg "enjoys music greatly," Williams said. "When he hears the orchestra play he sits and enjoys it, as though he were at a concert."