In animation, sometimes they add gags or awkward moments that completely break the pacing in order to fill the time of the episode. Other times, they cut too much.

The same happens to a lot of beginning writers as well, but I can't conceive of any reason why. There is no time to fill, no word number requirement, or even word constrain, and yet, many authors feel the need to add filler.

Maybe it's because they feel the pacing is too fast? The story is too short? The romance is too quick? And they are either too lazy to figure out how to expand it, or simply just don't know how?

I'm not entirely sure why, but please don't add pointless stuff to your fic because you think it needs to have a certain length/pacing to it. Only add things that actually contribute something to the story. I may write a bigger lecture on this topic later.

For now, have a hilarious example of what not to do:

The Best Words,
PiercingSight

Learned this in my native language lessons (Danish).

Take a movie like American Beauty, for example. Every. Single. Scene. Contributes to the movie and the story as a whole. There is humor alright, but it's made in a way, so no sentence or action is pointless. This is a reason why I dislike any filler episodes in any show. If the story writers can't make it span (for example) 22 episodes - why not add more depth to the storty rather than making pointless entertainment?
Rant over

I agree, and especially beware of "character must visit every one of the mane six, even if they have nothing to contribute." But I'd also warn to be careful of some things that might seem like filler, because they don't advance the plot, but actually have important places in as story. If your pacing is off, these are often a good additions that might fix it while contributing to the story:

Pacing on jokes: Basically, if you're writing a comedy fic in your head, and the punchline is "finally, Rainbow Dash finds Pinkie Pie, and asks her what all the umbrellas are about, and Pinkie says " Everypony knows that if you bring an umbrella, it never rains!"" In order to earn that finally, you need to show Rainbow asking two or three other ponies, or struggling two or three other times to figure out the umbrellas with escalating annoyance. If she just sees the umbrellas then asks Pinkie, that's not really a punchline.

Building character or investment in relationships:
Especially with an OC or a background pony, giving them a scene doing something normal towards the beginning is actually a really good way to show the readers who they are. I've noted that in the original Star Wars there are exactly two scenes that don't advance the plot: one is Luke eating with his aunt and uncle, the other is Han shooting Greedo. Both establish character for the characters who have arcs, the Han scene especially well.

Along similar lines, I've said many times that the best way to get people on board for the couple in a romance story is to show them having a normal conversation about everyday things. You get a sense of their dynamics and affection or chemistry. The same is true or any group of friends, really. Want me to believe your buddy cops have become buddies? Show them at the bar together, or in the squad car talking about a movie.

Reaction:
If something is wrong with your pacing, chances are you're not giving you characters a chance to react. I've had this problem myself. If you need to slow things down, especially in an action story or romance, give the characters a scene to process what's going on and bounce it off of another pony, show their emotions, and discuss possible paths forward.

6052978
This is very helpful for me, thanks for makin' this