The Dark Half

The author of highbrow literary novels under his own name, Thad Beaumont (Timothy Hutton) is better known for the bestselling suspense-thrillers he writes under the pen name "George Stark". Beaumont wishes to retire the Stark name and symbolically buries Stark in a mock grave.
However, Stark has become a physical entity (also portrayed by Hutton) and is terrorizing Beaumont's family and friends after he emerges from the grave. Stark then kills local photographer Homer Gamache and steals his truck. He also kills Thad's editor, agent, and his agent's ex-wife, and kills a man named Fred Clawson, who was trying to blackmail Thad for being a con artist that should not have written books under a false name.
When the police suspect Thad of murdering Gamache, he tries to convince Sheriff Alan Pangborn of Castle Rock, Maine he had nothing to do with it. After putting an all-points bulletin on Clawson, who was accused of the death of Homer, the New York police find him castrated and his throat slit. They find a message on the wall, written in Clawson's blood, "The sparrows are flying again." Thad starts to think that he may have a psychic connection to the killer.
While in his office, Thad begins to receive messages from Stark, and begins to worry about the next victim. He and his family start to receive threatening phone calls from Stark. Pangborn initially suspects the phone calls are a prank by Thad until Stark begins to describe how he is going to kill Thad's family, disturbing Pangborn.
State Police find Homer's truck with Thad's fingerprints all over it. For some reason, Stark wants to live after he appeared in a set of Beaumont's best selling books. Beaumont writes, but he is not alone in suspecting something strange: Sheriff Pangborn is equally suspicious and continues investigating. Thad begins to realize that Stark is, in fact, his twin brother who died at "child birth."
His mother never told him about the twin, until a local doctor tells him that Stark is a fraternal twin that was living inside Thad's brain. (A scene in the film's start shows a developing fetus inside Beaumont's brain). Stark arrives, kills the doctor, and blames Thad for the crime. A local friend realizes that Stark is an entity controlled by the books that Thad wrote, and that he will do anything he can to stop him. Stark kidnaps Thad's wife Liz and his children, and makes a deal with Thad: Finishing a book that has Stark living in the real world, or he will kill his family.
While writing the book, Thad notices Stark is healing himself with his own writings. Thad and Stark get into a fight, which ends with Thad stabbing Stark in the neck with a pencil. Thinking that it is over, Stark decides to kill Thad's children instead. Sheriff Pangborn arrives and unties Liz, who says that Thad and Stark are upstairs. However, a huge flock of sparrows comes and tears Stark apart, and take him back to hell where he belongs. The sparrows are agents of Satan, that come to collect evil souls that were not allowed to live. Thad and Liz are spared, and they along with Pangborn, watch as the sparrows disappear into the night.

Question: Who realizes that Stark is an entity controlled by Thad's books?
Answer: A local friend