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I DON'T think it is okay to introduce a second POV in what is basically the third act and approach to the climax of the story. I think that will throw readers. But the solution, as you are wonding...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/48607 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/48607 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
I DON'T think it is okay to introduce a second POV in what is basically the third act and approach to the climax of the story. I think that will throw readers. But the solution, as you are wonding, is relatively simple, introduce your other POV early on the book, just to "prove" to the reader that it can happen. Invent a reason to do that. Find some chapters that can be told just as well from this second POV, and rewrite them that way. The main reason for NOT writing a chapter from char "B"'s POV is if you want something to happen to char "B" and you _don't_ want to tell the reader what they were thinking or doing or feeling (since you are writing in 3PL). Of course, if char "B" has a secret they would naturally think about often, then you just can't write from char "B"'s POV, without cheating the reader. (Readers expect the deepest level of POV "access" you have already granted them.) You say you have already identified points, but you don't want to do so unless "necessary". Well it is necessary, to get the reader accustomed to the fact that POV can switch. Otherwise, it looks, not exactly like a deus ex machina, but something similar; you are springing something new on them, and it looks like you did it to "save the story", it breaks reading immersion. In the chapters of the first Act (roughly 25% of the story) readers are ready for anything, POV changes, magic, aliens, alternate universes, zombies, immortals, people that turn into dragons, whatever. They are open to all things. But from 0% to 25% of the story, their tolerance for new things diminishes from 100% to near 0%. That includes switching POVs, how much the narrator knows about the past, future, character's inner thoughts and feelings, all that stuff. If you are switching POVs, it is absolutely necessary to demonstrate that early on. Once you have done it, you can introduce NEW POVs throughout the book; but after the first Act, understand the reader's tolerance for "new rules" has diminished considerably. Finally, I am not saying it can't be done or hasn't been done by famous authors. People that write best-sellers have more leeway than beginners with no track record; just because Stephen King or JK Rowling did something doesn't make it okay for beginners to do something. They have strengths that compensate for their failures that other beginners do not necessarily have. I am saying it will be a red flag for agents and publishers, and they generally read (or have a professional reader read) an entire book before they make any decision to publish. So follow the basic rules of story, and introduce changing POVs early, and reinforce once in a while throughout, so this 3rd Act change feels natural to the reader, not disorienting.