Ah, sub-plot, one of my favorite friends. I absolutely love reading a book and getting hooked into a sub-plot; especially when it goes on for a series. When I'm writing, I want to capture that essence, that depth that sub-plot offers a book. After all, isn't sub-plot just a layer of story that lends credibility and realism to your characters? We all have sub-plot in our life. The trick in writing, is finding the right balance- not too much, but not too little.
We all have those friends- the ones when asked, 'so, how are you doing?' can go on for hours about everything in their life- right down to their feelings on second cousin Mavis' best friend from high school who may or may not have had a giant mole removed from her left buttock. Obviously, MORE sub-plot than we want or even need to know about right? The we have the silent ones- who answer 'fine'; then, a week later punch out their sister in the middle of a fancy restaurant because she caught her cheating on her fabulously hot husband a year ago and just can't take the guilt of knowing anymore. Probably could have used a teeny bit more sub-plot with that friend. Anyway, I give you these examples of real life- or what could be real life, to illustrate a point. Your characters need sub-plot to be interesting. I don't know about you, but I would have loved to have the guilt rolling around in the character's thoughts about what to do with the knowledge of the sins of the sister. It makes her real, and we, the readers, ache to make it all better. We know what she should do, and silently urge her to speak to her sister, then, when it blows up a the restaurant, we share in her pain- and the instant guilt of airing such dirty laundry in a very public place. It adds depth and substance to our story.
So, you say, of course I want my characters to have depth and my readers to feel a connection- but how do I create this feeling- and how do I know if I have too much or too little??
So, let me address the second question first- and I'll say, I'm a poor judge of this normally. I will, unfortunately, or fortunately for some of my friends, sit and listen until the cows come home about all sorts of weird crap and odd relatives. I find it fascinating. However, I know I'm the minority. So, for the question of too much or too little- I rely on beta readers- aka, my friends and my cronies in the Writer's Asylum . They will tell me with all honestly- where they stopped caring and started thinking about what to make for dinner- but even more so, where they wanted to know more. I also can tell most often, since I've trained myself, to look for the more/less factor when I'm proof reading. It can be tricky, but definitely doable.
As for the first question- you may already have this nailed, but if you find your character's lacking, here are a few tricks.
1. Create a diagram of the main characters and their relationships. Build some friction from those relationships. I love me some white/wipe- board action. Storyboarding with circles and bubbles is the bomb!
2. Interview some characters. Along the way, you'll find out something interesting. Maybe your leading lady has an intense fear of spiders and somewhere in the story she has to go into a creepy basement, full of the nasty little arachnids. And, maybe she has to go down there with someone who she doesn't want to know about her fear- say a hot crime fighting partner who might find it a weak attribute or maybe her arch-nemesis from college who will blog it all over the Internet. Use this one characteristic to build a sub-plot into your story.
3. Use a plot generator. Depending on your story type, and what you have going on, it can be fun to use a quirky plot generator for some odd sub-plots. I really enjoy using these during NaNoWriMo as well. You can find them all over the Internet- and there are some posted links here on the side of my blog.