When the main characters try to destroy the Lawn Gnomes, they reform like The Terminator. It is implied that all of the monstrosities from the books are like this. Why can they not be destroyed? It's because You Cannot Kill An Idea!
And the reason you cannot kill these ideas in the movie is that they're made of ink, which reforms their bodies no matter what is attempted.
Slappy's eyes go from blue to brown in the movie. Jack Black, who plays R. L. Stine, also has brown eyes, which is perfectly reflected in the mirror scene.
Slappy's prey of choice in the books is teenage girls, yet he never once speaks to or even takes notice of Hannah. Why would he? He must know she's not real/human, so where'd the fun be in that?
Why wasn't the Invisible Boy sucked back into the book at the end? Because the one encounter with him was so relatively inconsequential that Stine forgot to include it in the book. He can't be sucked into the manuscript of a Goosebumps book that he's not in.
Also, The Invisible Boy is just an invisible human who's a bit of a prankster. Being invisible and a prankster doesn't quite make one a monster, does it?
Alternatively as he was thrown into the field a while away from the other monsters, he may have just not been close enough to the book to be sucked back in when it opened. He is still a normal human child, after all. (This is supported by Champ closing the book after Hannah and stopping the process, meaning he might have shut it before all the monsters were inside.)
Also alternately, he trapped himself in the glass case where the typewriter is kept, and thus the book couldn't get him.
Another alternative is that the Goosebumps Book of the film has less power than the other Goosebumps manuscripts because Zach finished it, and not Stine. Stine implies that it isn't "just" him that can produce living monsters: Zach finishing the book might have weakened its Deus ex Machina abilities to merely a strong vacuum, rather than an Instant-Win Condition.
Also, Zach's story ending stipulated that none of the monsters would ever be 'seen' again. An invisible monster has a loophole there.
It could be a combination of all of these: Stine forgetting his brief bit, Stine mentally thinking through the monsters (accidentally skipping the non-monsters), Invisible Boy being too far, then Invisible Boy locked himself in the case afterwards, Zach finishing the book reduced the range, and then there's Zack's bad phrasing.
Slappy and the Invisible Boy are both voiced by Black. They're both extensions of Stine, so it makes sense they'd sound like him. Hannah however is an extension of the love he had for his late wife.
Fitting that Slappy chooses The Blob That Ate Everyone to help him kill R.L Stine. What better way to do in a writer of monsters than with a literal monster writer?
Champ lives up to his name after he saves the girl he likes from the Werewolf of Fever Swamp. He became her Champion.
And he did so by champing down on the Werewolf's neck with his silver dental fillings.
Unlike most of the Goosebumps monsters, The Invisible Boy is capable of speech, which showed when he voiced his dissatisfaction at how Stine defeated him. Unlike Slappy, Hannah, and most of the Goosebumps monsters, The Invisible Boy is just an invisible boy. He's still human, just invisible.
Why do some of the monsters seem a lot more vicious or malicious than their original book versions? Because these aren't oblivious characters with nothing but the motivations, personalities, and memories Stine wrote for them, randomly sucked into another world with no idea where they are or how they got there — they're fully aware that they're characters in books, angry about being locked up, and want revenge and freedom.
Alternately, it could also be because they're based on Stine's original manuscripts, filled with whatever raw emotions he was feeling at the time. He likely toned the stories down to get them published; he does admit one of his motivators is money, after all.
For that matter, why do some of the monsters seem more powerful? Because they have a new commander. Stine either didn't consider the full potential of their powers or perhaps simply wasn't comfortable with writing it. Slappy doesn't hold himself back at all and has spent a lot of time thinking up ways to destroy things.
It could also refer to how villains, no matter how scary, in media for children, rarely use their abilities to their fullest, to keep the story suitable for the audience. Free from these restraints, the many, many Goosebumps monsters are a lot more dangerous.
If all the monsters were released from their books, why isn't The Masked Mutant visible in the large group shot of them. Simple: Apart from his Voluntary Shapeshifting, he has no other abilities. He was likely disguised as one of the other monsters, as this would have made him more useful to Slappy.
Getting rid of all the monsters required writing a single Goosebumps book that would capture them all. R.L. Stine may be writing incredibly fast (writing an entire book in a few hours?), but he had all the inspiration he needed - he wrote the adventure he just went through. Of course, it helps that this adventure contained multiple moments of quick scares similar to the endings of chapters in regular Goosebumps books! "Something grabbed Hannah? No, wait, it's a statue" is only one such example.
The name Hannah is a palindrome - as in it reads the same backward. It turns into a Meaningful Name for Stine's daughter since she's only a character in his books, who has an endless loop of sixteenth birthdays.
Furthermore Hannah is presented with curly hair and a pale complexion which lets people know she's a ghost. In the end, when she is released from the book, she has a tan and straight hair, revealing she was rewritten as a human.
The way Hannah is revealed, she is literally The Ghost Next Door.
It is made pretty damn clear that the monsters cannot be killed, only trapped within their manuscripts. The And I Must Scream implications are horror in and of themselves, but it gets worse when you realize that Hannah is effectively immortal, and will outlive Zach, Stine, and Champ. Stine also burned her manuscript, so there is no way for her to die unless someone writes another story for her, which means her two fates are either immortality where she watches everyone she loves die, or to be forever trapped in a storybook like the others.
It's possible Stine realized this and wrote the manuscript in such a way that Hannah is no longer a ghost and is as real as everyone else, including actually being able to age and die.
Not how it works. You can’t write something into being real but it may be a good thing as there is a good chance that she may wind up developing the Living Forever Is Awesome mindset and it means that at least somebody will be around to deal with any more supernatural threats, which is good for the world as a whole.
He may not be able to write her "real", but he can certainly write a sequel in which ghost-Hannah is brought back to life, and sets out to lead a normal life in which she grows up, ages, and eventually dies like an ordinary living person. Perhaps the new book's ghost-Hannah can muse about how much she wishes she could do that, all through the book, and at the end, she receives a wish from some magical (and not cursed for once!) knickknack.
Or have said knickknack be cursed anyway. She would see it as Cursed with Awesome.
The twist at the end is pretty terrifying if you think about it. Stine passes his typewriter, sees it moving on its own, and watches as it types out, "The Invisible Boy's Revenge." The Invisible Boy implies he's going to do something to Stine himself. Roll credits. Typical cheesy Goosebumps ending, right? Except Stine has shown how easy it is to simply write another book to bring back a character, as he did with Hannah. What's stopping the Invisible Boy from bringing back Slappy and all the others?
Presumably it depends on whether or not the Invisible Boy is any good at writing.
When Slappy escapes his book and ends up facing R. L. Stine, he asks about what kind of trouble he's been let out for this time, and one of the things he mentions is destroying a town. Knowing that Slappy and the others were created as a form of therapeutic revenge that became literal breathing monsters, and seeing the events of the film, it's safe to conclude that other towns probably have been destroyed before and that Stine's continuous warnings of "bad things happen" when a book is unlocked make him Properly Paranoid. What's less obvious is Stine likely did it on purpose before. The fact that Slappy had a whole list of potential mayhem to cause speaks volumes of the kind of vengeful preteen/teenager that R. L. Stine used to be, even if he regrets it now.
What about all the Artifacts of Doom R.L Stine wrote, like the camera from Say Cheese or Die? Is it possible for them to be left behind like the Invisible Boy, since they aren't technically monsters?
Let's just hope Slappy'd stuffed them all into the trunk of the Haunted Car for ease of transport.
In any case, at least some of those things were in Stine's house, not in books, and Slappy seems like he'd only go after the books.
We should hope that Slappy only released the more dangerous monsters like Vanessa and The Lord High Executioner when he wanted to break into the school. If he released them while he was taking a drive around town in the beginning...
They had explosives in a school and, somehow, Zach knew how to use them to set up a bomb.
Hannah goes back into the book along with the other monsters due to being a creation in the book, even though R.L. treated her like a daughter. However, R.L. did not have to do this, since he was shown listing each monster by name, and at the end, it turns out that Stine forgot to include The Invisible Boy, so he could have just left her out of the list of names.
Questionable. Stine said it had to be a true Goosebumps story, so he couldn't cop out on the narrative. Until the end, the Invisible Boy did nothing important. Hannah was fundamental to the narrative.
More importantly, the young protagonist has to undergo character development or face some kind of pivotal choice for the book to be a proper R.L. Stine novel. Zach was the protagonist, and having to finish a book that would sacrifice Hannah to save the rest of the town was his pivotal choice. Also "The Twist", when Hannah shouldered the responsibility for actually opening the book to spare him that burden.
