When a Subplot Takes Off with its Writer

Sometimes writing a subplot reminds me of a horse I used to ride. If I didn't pay attention, that horse would grab the bit between his teeth and take off with me at a flat out gallop. He nearly threw me more than once—which was, of course, what he was going for. For better or for worse, I've had more than one subplot take me on a similar ride.

Like a ride on that horse, a subplot can begin calmly--just long enough to throw a rider or writer off guard. For me, though, when the narrative journey experienced by a secondary character or characters becomes complicated, when things get sticky, my subplot can grab the bit and take off. If I let it go, it might become it's own story and turn into a thrilling ride. Or it might run me into a tree (like that horse tried to do more than once). Either way, this sort of messes with my original work in progress. This is the heart of the problem. Is reigning in the subplot the way to go? Hmmmm.

Writers: Have you ever had a subplot take off with you?

Readers: Have you ever become more interested in the subplot of a story rather than the main story line?