“In a new study, researchers found that the brain appears to prime itself for laughter when it hears someone laughing:

This response occurs in the area of the brain that is activated when we smile, as though preparing our facial muscles to laugh...
Because we usually encounter positive emotions, such as laughter or cheering, in group situations, whether watching a comedy programme with family or a football game with friends,” says [a researcher]. “This response in the brain, automatically priming us to smile or laugh, provides a way of mirroring the behaviour of others, something which helps us interact socially. It could play an important role in building strong bonds between individuals in a group.”"
Which is a clear enough explanation for the crazy body contortions that fifty extras perform in sync with the hero of the Hindi film while he sings his love for the heroine