On my website, I have a page of writing exercises. I originally designed them for use in character development, but they have also proven useful for writer's block.
Today I'd like to highlight a couple of my favorites. Any of them can be useful, but these two reliably provide an interesting writing experience, and good insights when it's all done.
Your protagonist and antagonist are each required to write a letter of introduction for your reader, describing themselves, their goals and motivations, and you.
(This exercise gives you valuable insight into the way your characters think about and describe themselves)
Your protagonist and antagonist each write a letter to a friend or family member (or you!) about the other.
(This exercise helps you gain insight into how your characters view their opposition)