Secret Window, Secret Garden: Two Past Midnight

It is a novella collected in Four Past Midnight (1990). A writer named Mort Rainey is accused by a strange man called John Shooter of plagiarizing a short story. As Mort investigates the claim, the situation escalates into violence and the mystery of Shooter's origins deepens.

Yes. Secret Window (2004) starred Johnny Depp as Mort Rainey. The film was a faithful adaptation that worked well as a psychological thriller; Depp's performance received generally positive reviews, though the film as a whole was considered a modest effort.

The story is partly about the anxiety of influence—a writer's fear that their ideas are not wholly original. King has written elsewhere about the unconscious borrowing that all fiction involves; this novella literalizes that anxiety as an external threat that turns out to be internal.

Both Secret Window, Secret Garden and The Dark Half (1989) deal with writers whose fictional alter egos become dangerously real. King wrote them in close succession, and they share a preoccupation with the shadow side of authorship. The Dark Half is a fuller treatment of the theme.