How is George Stark related to the twin?

During the course of The Dark Half we learn that the killer is actually George Stark, an alter ego that Thad Beaumont invented in order to publish books not to be brought in relation with his name. When Thad decides to put an end to this alternate carreer, this alter ego turns real, though more as an incarnation of Alexis Machine, the antihero from George Stark's books. He seems to embody something like Thad's dark evil side and tries to push Thad into writing a new story about George in order to make him completely real. But then there is also the story about the "tumor" that Thad got removed from his brain when he was a child, which is revealed to actually have been the remnants of Thad's unborn twin that Thad absorbed into himself pre-birth.

But I am not completely sure how to bring those characters into relation. If Thad's evil side was bound to and removed with his unborn twin and now comes to life, then why is this George Stark whom Thad only invented years later, and why didn't it break out earlier? But if it is George Stark representing Thad's hidden evil side and created by him, turning to real life after Thad's attempt to metaphorically "kill him", then how is he related to the twin? Why even need the story about the twin then if he isn't related to Stark at all (though the sparrows, which are repeatedly used as "soul escorts" in the story, appearing at the surgery suggest that even back then the twin already had some kind of soul or a part of Thad's soul)? So what is the connection between George Stark and Thad Beaumont's unborn twin and what is the real nature and background of the killer? Did I miss anything in the movie or is this explained further in the novel?

And as a side question, why is it that George Stark's real incarnation is actually more like Alexis Machine, Stark's book hero, but wants to turn to full life by Thad writing about him, while Stark has never been a book figure but someone writing books? (Though, that may just be a minor iconsistency/mixup, but could also have a better explanation or be related to the above questions.)

Best Answer

This is spelled out much more explicitly in the novel.

The connection is that Thad's parents insisted that the pieces of the unborn twin that were removed be buried in the Beaumont family plot. Years later, when Thad metaphorically kills George Stark, he does a photo shoot with a fake tombstone -- right on top of the family plot. Just before Stark begins his killing, Thad is notified that someone desecrated the cemetery plot, digging a big hole right where the photo shoot had been. It's observed that it almost looks more like someone had dug their way up rather than down.

There's a common theme/trope/belief to the effect that naming something gives it power and that belief, if strong enough, can reshape reality. (This is at the heart of the expression "speak of the devil", to give you an idea of how far this idea reaches back.) Stark may have been meant to be only a pseudonym, but Thad treated him like a separate person. He created an entire backstory and history for him, changed his writing habits entirely when writing under that name, and even adopted aspects of Stark's personality when he got really deep into his novels.

But where did Stark come from in the first place? One possibility that is strongly hinted at is that Stark was his twin the entire time, and that the essence of the personality he created was really aspects of his twin that he absorbed into himself. This would explain why the sparrows were at the hospital in such numbers when the twin was excised -- they were trying to carry away the twin's soul, but couldn't find it because it had already been absorbed into Thad.

This can be backed up more by noting that Thad's books under his own name never did anywhere near as well as those written by Stark. Stark was the better writer, and when he begins to take over writing the book, he becomes stronger and Thad weaker, as though he is taking back what Thad originally absorbed from him.

So, putting it all together, the implication is that by staging a photo shoot on top of his twin's remains, a twin whose basic personality he'd taken into himself and then nurtured over the course of years by feeding it the power of his belief, Thad's rejection of that person he'd treated as independent unwittingly pushed the essence of George Stark back into the twin's remains, which then grew into Stark, who dug his way out of the grave and proceeded to kill everyone instrumental in his "death".