Viewpoint is an ongoing source of concern for writers. When I edit manuscripts, I comment on viewpoint problems more than any other single issue. Why? It’s really not that complicated (after you’ve done it thirty or forty times). Look, you just need to pick a viewpoint and stick with it throughout a scene. If you change viewpoint characters, don’t do it in the middle of the scene. No “head-hopping,” that is, drifting from one character to another’s thoughts and observations. That leaves the reader feeling dislocated. They don’t want to hear from an omniscient narrator who knows everything about everyone. They want to forget about the author/narrator and be immersed in the story. Readers will care more about your characters and what happens to them if you can get them inside the characters’ heads.
Establish viewpoint in the first sentence of the scene, whenever possible. Make the viewpoint character’s name the subject of that sentence. Then never use it again. Thereafter, refer to the character as “he” or “she” (which means no one else can be “he” or “she” in that scene, lest readers become confused). Any time you use the name thereafter, it tugs the reader out of the viewpoint. It puts them on the outside, looking at the character, rather than being inside the character’s head. It becomes more omniscient, and you don’t want that. You can also use interior monologue–that is, the characters thinking to themselves—to subtly remind the reader of the viewpoint throughout the scene.
This does restrict what you can write about, or at least how you go about it. You cannot reveal information your viewpoint character does not or could not know. If you’re in your female protagonist’s viewpoint, you may say something like, “She thought his smirk suggested he didn’t believe her,” but you cannot say, “He didn’t believe her,” because she doesn’t know. Internal monologue can fill this gap, but that doesn’t mean you should do it to excess, and it certainly doesn’t mean you should use internal monologue to tell rather than show. And fyi, you don’t have to use italics every time your characters think to themselves.
If you read any of the modern popular fiction superstars, like Steve Berry, you will probably find the viewpoint character identified in the first sentence, often as the first word of the sentence: “Mary walked into the room” or something like that. This is a solid, dependable technique. And then you refer to them by “he” or “she” for the rest of the scene. If you find there are so many characters of the same gender you fear readers will be confused unless you use the name again, put the name in someone else’s dialogue: “What do you think, Mary?” That eliminates confusion without pulling the reader out of the viewpoint character’s head. Maintain this tight psychic distance to keeps the reader firmly inside that viewpoint. Another technique for eliminating confusion is to refer to other characters of the same gender by a nickname or description, such as “Bozo” or “the tall dark man.”
Head-hopping, or stream-of-consciousness writing, is not as immersive, and omniscient narration has fallen out of common use–for a good reason. Readers don’t like it as much. What they like is to experience a story through the eyes and ears of one of the characters involved. Even when there are multiple viewpoints, readers should have a sense of who the main character is and should spend most of the time inside that head, experiencing the story as the protagonist does.
The usual reason for introducing additional viewpoints is to increase suspense. The narrative flips to an antagonist’s viewpoint and the reader learns something the protagonist doesn’t know yet. This creates tension, because the reader knows something bad is in the works but the lead character doesn’t.
One last thought: many beginning readers try to avoid these viewpoint problems by writing in first-person narration. I do not recommend this. If you think first person is easier than third person–you’re wrong. It is far more challenging to do well, because it means constantly staying in the voice of a character–who should not be a sassier, braver version of yourself–for an entire book. While it can be done, I think it’s best reserved for writers with experience and a clear view of their character’s unique identity.