The Dark Half (1993)

As a child, Thad had headaches which were believed to be caused by a brain tumor.  When doctors operate, they find that it isn't a tumor, but an unborn twin (only an eye, a nostril, and a few teeth exist).

Now an adult, Thad teaches at a small town college and is a noted (but not well selling) author.  But he has a successful writing career  under the name George Stark, who writes violent detective novels.  After a student threatens to reveal his alter ego unless he gives him a payoff, Thad decides to reveal his secret and thus eliminate any blackmail attempts.

Shortly after the article and photo shoot where Thad declares his  alias to be dead, someone starts killing everyone involved with the reveal.  Evidence points to Thad, but he insists he's not the killer.

Timothy Hutton does a fantastic job in a duel role. He was so good I wondered if it was another actor made up to resemble him.  While the physical changes are minimal, the difference in persona and swagger is impressive and shows Hutton's talent.  In a lessor actor, this role would not have been handled as well.