Applies to all fictional writing
Protagonist and your own Opinions
Support your characters - even the villain
Personally I'm not a fan of things written in the first person. There's a little too much assumption, a few too many confused thoughts, things like reactions go by unnoticed - it's just a mess. I'm not a fan. I usually stick with third person. I find it easier to switch viewpoints that way, but the purpose of this segment can easily be applied to both first and third person, and genres beyond fantasy.
Viewpoints are very important. You can follow a single character's thoughts and feelings, but it won't give the story enough depth. You will probably find yourself trying to write from the view of a character other than your lead, such as the villain, or a subprotagonist, and it is extremely important at these points to be versatile.
You will probably also find that your lead protagonist has a lot of similarities to yourself. You'll follow them closer than any other character, and while it could be accidental or intentional, you may well relate better to them than anyone else in your work. At these points your protagonist will probably also share the same opinions as yourself (a habit you won't want to get into). While it will make it easier to write and easier to explain the reasoning behind such opinions, when you switch viewpoints to someone else, they cannot carry the same personality, viewpoints, opinions and train of thought as everyone else. As such it's best to try to keep your own viewpoints completely out of the equation.
You don't want your characters to all become the same person. I've read books that I'm still quite fond of, but I noticed while reading that a lot of their characters all sort of bonded into one individual, with the same ideas, same train of thought - even the same humour. I still love the books, but it was a big flaw. If it weren't for the intricacy or genius of the stories themselves, it would have caused me to put the books back down.
Remember that some opinions may be shared by characters, others may differ slightly, and others may be complete opposites. Even the protagonist and their best friend are likely to agree on one thing and not another - the protagonist and the antagonist could also agree on a few things. They don't have to be complete opposites. But even if some things are agreed upon, they may not have the same motives, regardless of who it is, and will probably also have different trains of thought.
Keep in mind how different individuals can be who agree on things. Sometimes I can agree on something with Seeg, but we have very different ideas on why we agree on it, and how we got to that point. I'll give a really poor example here, but it avoids arguments on religion and politics: we both love pizza, we really do, but I love cheese and tomato with thick crust, while he loves meaty ones with crispy crusts - we both love pizza, but not for the same reasons. Told you it was a poor example.
Viewpoints can help to convey the truth behind happenings, characters' own paranoia and whether or not the antagonist is succeeding with their plan on a more mental or disruptive level. Viewpoints can bring things to the reader's attention that other characters may not have noticed. For example, if Character 1 were to tell Character 2 some great news, C2 may well not notice any look of uncertainty on C1's face, while C3, also present while the information is passed and perhaps less affected by the news, would notice it. More viewpoints are like having more eyes, more trains of thought - more points of view.
Sometimes it's used to replay the same scene. First you might see a person murdered in C1's view. They may not have seen the killer, but they heard the scream and found the body. You can replay the scene for C2, who may not have seen the killer, but they too heard the scream, and when they got to the body, found C1 covered in their blood, and jump to conclusions. Replay again, and C3 sees the killer and knows C1 did nothing - but they may or may not be a good person themselves, and so may or may not correct C2. Replay again and C4 is the killer.
Alternatively, you could replay a scene all four were present for, but that would mean replaying even the exact scenery over again. In these cases it's best to only revisit the scene several times if you have a damn good reason - each character notices something different, but important. Perhaps C1 hears words of someone else echo in his mind which suddenly become significant, C2 sees something on the ground or some kind of a trail or clue to something, C3 knows who is behind it or has suspicions, but none of this information is shared among the three of them for one reason or another. The scene may also not be revisited consecutively. The scene could take place in chapter 3, it's revisited in chapter 5 and then again in chapter 10.
If you replay your scenes, they must only be replayed for good reasons to avoid boredom, and the further you space out these replayings, the more significant the scene seems, so if it isn't the most important thing, but still important, don't space it out too far, but if it's basically the pivotal point in the book that causes all of the events, but appears very soon in the book, then by all means, replay it across twenty chapters, but don't bore your readers.
As I've said before, while viewpoints can be used to point out things otherwise not noticed, they will also portray different character's ideas and impressions. As such, you will have to remain neutral. You're the story teller, the narrator, not a character yourself. You must leave your opinions and thoughts out of it, and be able to give other characters opposing thoughts. Even if you absolutely despise your villain, even if he is the most wretched individual you could possibly concoct, while you are writing from his point of view, you cannot write with an untrusting tone. He believes that what he's doing is right, and that it makes sense, and that has to come across in the narrative. Don't make him a big fat doody head with onion ears just because you want the readers to hate him - let them come to their own conclusions, show them why they should hate him with his own personality - or perhaps you want them to like him just to make the story more interesting. Either way, you must be able to support both the protagonist and the antagonist. This applies to first person, too.
A note on first person, however: first person is more difficult if you're trying to follow other individuals. This is another reason why I prefer third person. Whenever I've seen first person it's only ever been from the point of view of one person, except one book. I don't remember what it was called or who it was by, but I explicitly remember that it was split down the middle. Every chapter alternated between two individuals, one was the protagonist, the other was the antagonist. I thought it was quite clever. Unfortunately, it's a little more difficult to make the switch stand out, because a character isn't likely to proclaim their name as often as a third person narrative would. But when the chapters alternated, the protagonist's chapters were written normally, while the antagonist's were written in italics. It made the difference much more obvious, though I'm not sure it is necessarily something that would be encouraged by a professional, a teacher or whatnot. But either way, I read the book, I enjoyed it, even if I don't remember what it was called.