Who says acting is easy? Not when you have to carry a boy weighing 45 kgs on your shoulders. Just ask actress Vidya Balan, who had to go through the exercise for her film "Paa" -- and ended up falling.

"In a scene, Vidya had to shoot with a child who is playing the younger version of Amitabh Bachchan in 'Paa'. The scene required Vidya to carry him on her shoulders, " said a source.

"Vidya weighs 45 kgs and so did the boy. Vidya carried him on her shoulders nevertheless. Suddenly there was this loud thud and people saw that she had lost balance and fallen on the ground along with the boy. The actress still completed the shot, " added the source.

"Paa", slated for a Dec 4 release, revolves around a father-son relationship. While Abhishek Bachchan plays the father, Amitabh Bachchan will be seen as his son who is suffering from a rare disorder called progeria.