Wow, an article, must be something in the water.
Filler is a concept that I think really took root in the anime crowd. Anime is often based on manga but since anime tends to move faster than it's source material, the studios have to do something until they can go back to the manga's main storyline. This is filler, stuff used until the main story can come back into play. It can be used in other places, including this blog (see last week), but is actually pretty rare outside of anime.
And webcomics. It's actually funny that webcomics use filler because two of their common sources, comic books and newspaper comics, really don't. Comic books either have such a convoluted story that "filler" doesn't actually exist or they just delay publishing until the comic books is finished. Newspaper comics have something similar, called Sunday Comics, but it's less filler and more a second story line in many cases, and if an artist misses a deadline (VERY RARE) they simply run an older strip.
Webcomics, however, are different. There are no set schedules, few webcomic artists actually make a living on their comics, and if they do have a schedule, they usually are running right along the deadline. Thus comes the filler strips, comics that are usually noted as such and do something that is not artist like.
Filler comes in various forms, and I apologize now that I won't be including specific examples. I'd have to dig through a lot of comics to do that, and I really don't have the time or energy to do so. I'll at least point you toward some of the comics I've seen such filler in, if I can remember them.
The most common filler is the art post. Sometimes it's just a sketch, sometimes something more, but always a random image taken from the artist's files. Typically they aren't planned, though Sluggy Freelance has done it more or less consistently on weekends since moving to a five day a week schedule. I actually think this is the best filler of the batch as the artist still gets to put art up, even if it isn't a comic, and it doesn't interrupt or derail an ongoing story.
Sketched pages of the comic are also a form filler, but less common than outright art. A lot of reasons for this, mostly because many artists are perfectionists that would rather post nothing than something incomplete. At the same time, there are those who don't mind, such as the artist for The Meek. After each of his chapters, he posts (very quickly) the rough draft for it. The situations are often very different and worth reading if you get the chance to see them.
At some point, the artist will take more than a day or so off, and instead replace the comic with a sub-comic. Typically this will be in a simpler style (stickman) or just simply be different characters. Getting away from the main story, even in the middle of one, probably acts as a relief valve for the artist, giving them time to organize their thoughts and notes on the next big plot development. And probably helps the readers wind down a bit too. I again point to Sluggy Freelance because he does it most often, though he does make an effort to have the breaks come between storylines, rather than within one.
Guest strips or guest art are the next phase of filler. I often wonder if artists like guest art less for the fact that they don't have to draw anything, and more because of the ego boost it gives them. It's scary though when the guest art is actually better than the original comic, and I wonder if that hurts the ego thing. Still, that's rare enough that it doesn't come up much. The Wotch is running another string of guest strips, though the reason why still evades me.
Question and answer strips fall in around here. I suppose for the artist, Q&A gives them a chance to resolve story problems that the readers may have been having with the comic, but it is still filler as these answers could, and should, come in the main comic itself. Other times, it's not actually Q&A, but more information dump, an attempt on the artist's part to explain things that they know instinctively at this point, but the reader doesn't. Alpha Luna did this, and I consider it the worst part of the comic. I know WHY it was done (there is no room for explination in the action scene that was taking place) but it could have been done in so many other ways. Then, of course, there is Heart Shaped Skull's "Vicious Whispers" segments that are probably the most fun and creative Q&A sessions I've ever seen. I think it helps that the questions cover just about everything, and at the same time resolve certain points of the universe for the main comic without it being so direct.
Filler is something that webcomics have to deal with because they often don't have a team of artists, extremely deep archives or even the ability to branch into alternate storylines. At the same time, filler is NOT all that common in the rest of the world, so webcomic artists would be wise to try to avoid it as much as possible. Build up deep buffers of comcis, and if you must do fillers, at least get creative.
That's my thoughts anyway. See you next time kiddies.