When a fic takes on a life of its own...

For a while now, I've been working on my Grawpfic, and now I'm almost finished, I'm looking back in mild astonishment at how much the thing has changed.

It started out as a gentle plot bunny: "Oh, the fandom all hate Grawp for being pointless, so let's give him a point. And they hate him for being stupid, so let's make him clever. And there seems to be a lot of Luna- and Colin-hate around at the moment - so let's give them their moment in the sun, too."

That was all. The story was going to be a gentle comedy of about 8 pages in length. It's now standing at 33, and is going to run to several more before I've finished. And the ending isn't funny. Not funny at all.

The odd thing is, I had a definite plan for this story - all written down on paper and everything. But once I actually started to write it, things took a life of their own, and drag me down the oddest side paths, including a diversion into romance (subsequently quashed), and then an ending which was quite different from the one I'd originally planned.

I've not been in the fanfic business very long, so please can someone tell me: Is this normal?