I just sent out the first draft of a commission back to the commissioner. It ended up being just over 40k words out of a 15k word request. In other words, I blew past the word count and added almost three times the length just to get the story out. So, I think to say as a commissioned writer, it was a failure, but as a person who loves fiction and longer stories a general win.
So, how did things go so terribly wrong?
We'll start with the commission itself. It was original a 10k generic story about 4 friends who get on a roller coaster and end up in a magical world with a talking loli unicorn... wait, not quite. The commission was the basic X people have Y adventures in Z setting. Now, the number of people always varies in this commission, but for my commissioners it is almost never below four and I've had requests for twelve in a 5k word story (I said no to that one, they said how about 15k, I said yes). I don't know why, but this is always a multiple story and they usually want a good amount of character development for each one. The commission in question was five people. Now, I roughly need a thousand words to develop a character in a story and handle their individual "side quests" if you will.
If one of the character really resonantes with my own personal interests (in this case, "one of the girls gets a magic bow" and my obsession with archers), that makes it really easy to create their story, but then I feel the need to give the others an equal amount of development. (Despite not having an awesome archer powers. Seriously, I have a thing for fantasy archers.) In this commission, there were two such characters.
Thirdly, the story was the third in a series. The first two commissions were 18k and 12k respectively. Like most writers, it seems like the sequels just get larger and larger. I had the basic setting of the story set pretty well and what would be a logical conclusion ended up having a lot more detail put into it because it was so developed.
Finally, I moved. I know it sounds strange but usually I work on a commission over a couple weeks and I'm done. I get a fatigue, as it were, because of how I write which helps naturally come to a conclusion. In this case, I wrote a little, packed a lot, wrote a little, went to work, wrote a little, played with EDM. Each time, I get a refresh of ideas and new ideas. A simple scene that should have taken 200-300 words ended up taking 1000-1500 simply because I was playing with it in the back of my head.
In the end, I knew it was building up to blow past the word count. During the entire time, I was emailing the commissioner because I was behind schedule and it is polite. If you do commissioned writing (or art), giving status updates really helps with encouraging them to do it again. During the weekly emails, I gave updates including rough word counts.
Even with that, why did I keep working on it?
The first was easy: the story is as long as it needed to be. A cheating answer and one that has frustated me with FOTS because it was so long. But, I came up with a story in an established setting and it just... fit the request (which was a single paragraph, by the way).
Also, the commissioner is a repeat customer. A well-paying customer who likes my stories. In a side project that has erratic income, someone responsible for a quarter of my commission income in the last four years deserves a bit of slack. It will increase the chance they will use me again.
Probably the main reason is what I'd akin to interia. I got a great idea for the story and wrote it. As it started to get out of control, I felt the need to keep moving because I thought they would like it. So, I just moved forward and didn't worry about the word count as much. (Related, I don't give up on books either, if I start, damn it I'll finish it.)
I won't get paid for the extra words. I also won't be able to ever show this story to anyone else. Once I write it, it will pretty much cease to exist except on the commissioner's computer. It is a little flash of excitement and then it's gone. But in that flash, I wrote a story I could be proud of.