The Tragic Muse, which also bears the themes. It may speak well to James that he was so relatively reticent in writing a story about writing stories—that would be more of a 20th-century problem—but it's obvious in this story that writing fiction was the creative life he knew, not theater.
"interlocutor" count = 2 / 63 pages
In case it's not at the library. (Library of America)