What stops you from taking action?

I talk often about how to find "fourth options." But what happens after your innovative idea is conceived? What do some people take action on new possibilities while most hesitate on the sidelines? Three factors are at play:
1. Do believe it will work or not?
2. Do you believe you are capable enough to do it or not?
3. Would taking the required action be socially acceptable or not?

If you are not acting when you should be then one of these is holding you back. Which is it?