For a book or story to work, we authors try to create realistic characters. We need to know what they look like, what motivates them. If we know them well enough, we're able to write their responses and reactions to whatever thrown at them almost automatically - as if the character is controlling the words you type. Do it right, and the reader feels and sees what they do, is frightened or happy or whatever alongside them.
And we, as authors of horror, mystery, suspense, throw a lot at these people. Put them through the ringer as the expression goes. No one should have to suffer the way some of these make-believe characters do. In real life, they do, sometimes, sometimes not.
But what about when the last page is written and the book is closed? How do these people pick up the pieces of their lives and move on? King is pretty good in this regard. He tends to tie up loose ends, implies either implicitly or explicitly how the survivors of his plot will fare later. Koontz, of whom I've always been a fan, not so much. He ends with his characters, and readers, exhausted.
How do they cope, move on from the events of the story? If it’s a disaster-type scenario, some natural scourge against which the people have struggled, it's implied they move on with their lives, in a world that may or may not be the same. They'd fought, and survived, and are better for it. But let's take a murder mystery. Or better yet, a thriller. The hero of the story has defeated the bad guy(s), but not before "learning the truth" about her trusted butler, or great Aunt who's really an axe murderer, or employer who's secretly been working as a slave trader. She survives, makes it through, is safe again.
Then what? The next job she takes (because her office building burned down in the climax), the next man she meets (since her late husband tried to poison her with Playdoh or something equally diabolic).. does she wonder whom she can trust? How long will she relive the events covered between page 1 and page 423? If her best friend is revealed on page 397 as being a flesh-eating alien, how will she act towards friend B or C? What if they're also flesh-eating aliens but are hiding this fact because the good guys will find them and melt their wax like they did to A?
I can imagine our former protagonist sitting in a therapist's office, nodding enthusiastically when the doctor says she needs to move forward, focus on the present. Maybe she could, given time with no more tentacles jumping out of her pudding - but will she wonder if she's safe because it hasn't happened again, or because she stopped eating pudding without first donning a neck brace?
Habits have been born into her psyche, from solving the mystery and surviving the attack in the latter half of the book. She survived by seeing the clues chapter after chapter, denying what she was seeing at first, then having no choice but to accept something was amiss. Only then could she discover the plot before being eaten like so many others before her. But... long after THE END, she tries to surround herself with people and occupy her mind because when she's alone she thinks too much, finds herself seeing things that might not be there, or having suspicions that might not be warranted. She tries to look away because it might mean being dragged back through the same nightmare but she cannot ignore that trusting her instincts defeated the bad guy the first time, saved her from certain death. Is it real or some post-traumatic paranoia?
This could lead to a sequel, Jamie Lee Curtis crawling along the hospital floor because, finally, she acknowledges that Michael Myers is not dead.
If our beleaguered survivor was real enough, she would be plagued by worries that she missed something important - had her oldest friend with whom she defeated the evil alien boss been a part of it, implicit in the planned invasion in some way? She doesn't want to think this, but there had been signs, especially around pages 112 and 175 - she simply hadn't enough facts then to see it clearly. Even now, things feel... wrong (cue soft dramatic music). But it's over. She's safe now. She needs a friend, someone to go to the movies with to forget all the craziness that transpired. Besides, he couldn't have been that involved in such a despicable plot since the Great Author never gave him his come-uppance.... Could he? (cue louder dramatic music). Part of her doesn't want to know, but after five hundred pages filled with deception, truth is an important commodity. She shrugs, acknowledges that she would probably never completely know and because of that, never complete trust anyone again. Either way, the Great Author isn't going to write a sequel; has left her on her own to cope however she can. Authors are like that - they move on to the next story and try not to think much more about the characters they created. After all, these people aren't real.