You might think that choosing one character as the viewpoint character and sticking to them would solve any POV problems. Not necessarily. For example, if your first person or third person limited narrator doesn’t actually witness an important event in the story, then he or she can’t describe it.
Of course, in theory, you could use another character to tell that part of the story, but one of the main rules for POV is that it should be consistent. So if most of your story so far has been seen only through the eyes of one character, switching POV at this late stage may seriously disturb your reader. At the very least it will break their connection with the first character which has been building for most of the book and, once broken, there is no guarantee you will get it back again.
Imagine, for example, if Jane Austen had discarded Elizabeth Bennett’s POV following Lydia’s elopement with Wickham in Pride and Prejudice and sent us galloping off to London with Mr Darcy instead. Apart from the consternation of finding ourselves suddenly flung into the intimate company of a man seen to this point only through Lizzie’s eyes, we would miss all the tension, irritation, anxiety and feelings of helplessness that our heroine goes through during her long wait for news in Longbourn. The advantage of all this soul-searching not only adds considerably to the reader’s experience of Elizabeth Bennett’s character, but also ups the ante for the moment when she learns the truth about Darcy’s role in rescuing her sister. And who better to tell Lizzie about Darcy’s involvement, but silly, indiscreet Lydia? A delicious combination of plot point and character moment.
If your narrator finds themselves in a similar situation, you do what writers have done for centuries: you get creative. You have your viewpoint character talk to people, overhear conversations, read letters, newspaper reports, books, secret diaries or files (or their modern-day technological equivalents), basically whatever it takes to get the information the reader needs.
But no Deus ex machina, please. This Latin term meaning ‘god from a machine’, refers primarily to the Greek tragedy penchant for having gods ascend or descend miraculously in mechanical stage devices (hence the ‘machina’) at the end of plays to provide improbably contrived resolutions to unsolvable situations. Please do keep your POV solutions within the context and internal logic of your viewpoint character and the world of your story.
Most of all, you should view the limitations of a narrator choice not as a downside, but as a virtue and a truly wonderful opportunity to build up oodles of character, atmosphere, tension and plot. What’s not to like?