Not critizing anybody and accepting what other people tell you is the core rule of improv comedy. Instead of disagreeing with someone who talks about talking to his grandmother you could start talking about demonic possession and see how far the person is willing to go.

Leaving your own beliefs at the door and using another mindset for an evening can be a lot of fun. It also a good exercise to argue positions in which are very far of from the ones in which you believe in in a coherent manner.