I personally don't like convincing others to do things that they don't want to do.
I would only do that to a kid, assuming that he or she is just being stubborn or unwilling to try new things. In this case, I would either try to bribe the kid with a treat or do the thing that they don't want to do with them, so they won't have to be afraid.
As for adults, I would let them be. They're adults. They should know better. If they don't want to try something, it's probably for a reason. I would like to believe that they are smart enough to know what they like and be aware of the reasons why they refuse to try something. Insisting could just annoy them. And being manipulative and coercive will just anger them. So, to keep the peace, I'll leave them be.