When I was a little kid I used to play with guys twice my age so I was the last one picked so if I picked I knew that I had to get the ball to the scorer if I wanted to stay on the court so that was pretty much my job.
Most of the top actors and actresses may be working in ten or twelve films at the same time so they will give one director two hours and maybe shoot in Bombay in the morning and Madras in the evening. It happens.