An actor does what a scene demands. Arfi Lamba, in a particular scene in his upcoming film with Akshay Kumar - Singh is Bliing - was asked to go underwater. Lamba, who has been hydrophobic since childhood, conveyed his fear to the film's director Prabhudheva.
The actor-turned-director, instead of asking Arfi to not do the scene, helped him get over the fear of water by getting swimming coaches. They got some floats and helped Arfi perform the scene. Though scared at first, he was eventually able to get over his fear.