In The Writer’s Path, by Todd Walton and Mindy Toomay, writers get a few exercises (among many in the book) that use the letter form. Letters are more intimate, a way to get at style and voice that might be escaping you in third person writing, or the constraints of writing a novel, or a story, instead of simply telling a tale.
The book says that “the difference between stories written for publication and [those] written as a letter to a friend… may have no technical difference whatsoever. But the difference in our sense of love and trust for the people who might see or hear our words is enormous.”
So in one exercise we practiced tonight in the Writer’s Workshop, we wrote a letter to a friend about an interesting person in our lives, or in our stories. The writing came out with extra voice, compressed detail that did not seem forced, vivid images, and no affectation. On a revision, you might be able to use this writing inside a story or novel itself. At the least, it gives you a grip on voice for a narrator, as well as character details that are most important.