I've been in the writing world for over five years now and I've known for a long time that if characters can have a good, honest talk and work out their troubles, then the plot conflict isn't strong enough.
But I hadn't thought of this in the form of assumptions until a reader made a comment about a book and how she enjoyed that the characters didn't just assume things about each other.
A character's goal has to be impeded by more than her thinking the other character (antagonist/hero) wouldn't like the goal. The internal conflict should be more than her thinking the other character will disapprove or hate her.
I adore internal conflicts but they have to have a concrete external conflict to make them believable. If the conflict isn't believable, or worse, could be cleared with an honest conversation, then this hampers the tension of the story in a big way.
The reader might not care about turning the pages to find out what happens.
I know for a fact that one of my manuscripts relies too heavily on assumption as the basis of conflict. That's something I need to change.
Have your characters assumed too much? Do you like it in books when there's a misunderstanding or would you rather there be definite stakes/goals driving a wedge between characters?