Is a writer responsible for what her character says?

I was reading a review on Avengers: Age of Ultron the other day, a I ran across a comment from someone who was really angry about the film because of a statement one of the characters makes about not being able to have children. It’s one of the many heart-tugging moments in the movie where Natasha Romanov (aka Black Widow) and Dr. Bruce Banner (aka The Hulk) are …