I know you're all sick of me uploading at random (And then complaining about it), so I'll make these notes short and sweet. Today we'll be tackling the dreaded 4th Wall! Oh the joy *sarcasm*.
If you were smart enough to look at the title of today's installment of "How to Not Screw Up a Story in General" (A title so unnecessarily long that it would be a chore to abbreviate it), you'll know what I'm talking about. If you are a REALLY good reader and paid attention when I talked about this being on DA originally, you probably checked out that version and slammed your head against a wall. This rule on that version wasn't explained very well, so let's start over with the source: the 4th Wall.
The 4th wall can be a pain to deal with if you're switching POV's constantly, but it really shouldn't that much of a issue. Before the nitpick begins, let's go back to primary school and take a look at the points of view themselves.
1st Person: The author and the reader are the main character; you are talking from their perspective. This allows the reader to empathize with the character in question and is an easy way to make the story more interesting.
2nd Person: How many of you have heard next to nothing about this view? Well, I don't blame you, it's a weird way of storytelling. Unlike 1st Person where both the reader and author are the main character, only the reader is. This time, the narrator refers to the character as "You" rather than "I" or "He/ She". Again, it's rather unusual which is probably why it's used so little.
3rd Person: This point of view is the narrator's responsibility. It is their job to tell the story as a nonexistent force. This is where the problem with the fourth wall comes in.
So you're reading through yet another story, and it just so happens to be told in 1st person (because everyone knows that talking in first is way easier than any other point of view), then suddenly WHOA WHERE DID THIS NARRATOR COME FROM? It should go without saying that it would probably be bright if you warned the reader if you're changing POV. There's nothing wrong with changing the view, just be wary of the possibility of plot-holes.
Regarding every POV, you shouldn't be breaking the 4th wall to express your own opinions. This rule really shouldn't be hard to follow; if your story is told in 1st person, then they shouldn't even mention that they're in a story and they shouldn't mention what 'Universe' they are from. In 3rd person, then ACT LIKE A NARRATOR. I've never seen a novel where the author broke the 4th Wall. Tapped the 4th Wall, yes, but straight-forward breaking it? No.
Now if you're writing a parody, go ahead. Better yet, go nuts; parodies open a lot of creative doors. As for humor stories, if you want to "poke the 4th Wall", I won't stop you. Just be careful that you don't completely demolish it. You are telling a story, and breaking the 4th Wall breaks the immersion which effectively ruins the experience.