When using POV, the narrative is limited to the things that POV can experience. I.e., if you have second-person narrative, it cannot contain anything that someone else feels/sees/hears. The POV can observe the appearance of someone else feeling/seeing/hearing those things, but they cannot be told directly.
There are any number of excellent sources for POV (including this wikipedia article). However, while these often describe what POV is, they do not always discuss how to use it—or how to recognize when you've gotten into trouble and need to correct your mistakes.
Most stories are told in third-person POV, as this tends to work best for readers. It can be depersonalized enough to be used in factual writing (news-reporting, technical writing), or infused with opinions or characterization for fiction, op-ed pieces, and fan-fiction.
I've seen first-person recommended as the next-best choice to third-person, but this is more prevalent in original fiction (i.e., novels) than in fan-fiction. In fan-fiction, second-person is the next-most-common choice (though rarely used). With original fiction, the reader is being introduced to a character of the author's creation, and anything the author chooses to show us is assumed to be the character's true voice. With fan-fiction, the reader is generally already very familiar with the character and likely has a lot of opinions as to who that character is and how they think. This may be why first-person tends to be unpopular with fan-fiction readers— even though the writer could show all of that same characterization in third-person, and it could be just as authentic or off-base. It will still "sell" better to the reader in third-person, regardless.
Because third-person is the most common, the most varied, and the most problematic, this discussion will focus on the methods and pitfalls of using it.
Third-person has several different forms. It can be an omniscient narrative, where the story goes inside all of the important characters' heads. It can be a limited omniscient (character-focused) narrative, where the third-person POV gets inside the head of a single character. Finally, there is the objective narrative ("fly-on-the-wall" perspective), where no internal thoughts of any characters are revealed.
Regardless of which narrative form the author intends to use, problems occur if the story develops into a different narrative form during the writing process.
Frequently, when I write fan-fiction myself, I begin with the notion that the story is going to be third-person "omniscient." What happens, however, is that the story usually drifts into a character-focused third-person narrative instead. The story may have begun firmly on middle ground, but after a few pages it has solidified into a single character's POV instead.
Now, what should the author do when this happens?
First and most importantly, be aware that your story has drifted! This 'drift' absolutely must be fixed.
How does the author accomplish this? There are two approaches here.
One is to accept that the POV you drifted into is the way your story wants to be told, and stick with that (it's hard to deny a convincing narrative voice, even if you didn't expect it to be that particular voice). If this is the tack you take, then you'll need to go back through your story and revise all the places where someone else's POV was used. I.e., instead of
use something like
Or instead of a character thinking something, show actions that indicate this:
The other choice is to go back and expand the POV throughout your story to maintain the feeling that all of the main characters are 'present' rather than just a single one.
I have seen the flip side of this situation, both in a story I've been beta-reading and in a novel I recently finished reading. In these cases, the author intended to write omniscient third-person narrative but established limited-omniscient third-person as the POV instead.
This brings up another point: you must establish the omniscient third-person narrative (ideally as soon as possible) in order to use it.
The novel I read had two-and-a-half chapters of the main character's POV, and then drifted into someone else's head for a paragraph. Then it reverted back to the main character for another chapter, drifted briefly into someone else, went back to the main character, and eventually tossed in a few chapters in other POVs in order to complete the (already thin) plot.
Your finished product cannot "visit" other POVs in this way simply out of laziness (to save the steps of "showing" the other characters rather than "telling" them) or because you couldn't bring off your pivotal plot point without doing it.
Instead, you must establish the other POVs firmly from the beginning.
In a longer work, such as a novel, you could make the entire second chapter in a different person's POV, and go chapter-by-chapter with different POVs. The narrative break of changing chapters is why this technique works. Or you could choose to expand your narrative from the beginning, introducing the POVs of the other major characters (via phone calls or parallel settings, for instance, where the scene shifts to the other person's perspective on what's going on).
The point is, this has to be done sooner rather than later. If you mainly write from a single character's POV in third-person, any time you go into another POV it comes off as drift rather than a legitimate narrative approach.
The story I beta-read was a single character for the first half, and then moved into the other two characters to highlight fear and danger (character #2) and angst (character #3). Given how the story ended, where the emotional core of the final scene required the narrative to focus on that third character, the best approach was to expand the POV from the beginning of the story and solidly set it up as being omniscient. This was the technique used:
Paragraph One (original): Character #1's POV, in which the other two characters arrive.
Paragraph Two (added): the two other characters observe Character #1, and the narrative broadens the emotional context of the scene.
Ensuing paragraphs: add individual emotions around some of the second and third characters' dialogue, add Character #2 observing Character #1 and vice-versa, add Character #2 observing Character #3, etc.
In the revised version, by the time the story shifts into Character #2 facing danger head-on, the reader is now prepared for the narrative to move between characters. There is shift, but not drift.
That distinction is key.
In summary, decide which POV approach you're using and make sure that you adhere to it. Be aware of the additional care needed to set up the third-person "omniscient" POV, and to maintain it throughout your story—whether you sketch out the POV shifts ahead of time during the plotting or simply go back later and make sure you haven't settled too firmly into a single-character POV.
If you find that the single-character POV works better for you, it's all right to convert the rest of the story to it. So long as you're consistent, do whatever it takes to tell the best and most compelling story you have within you.