Buffy is a good example to use when looking at B Story because the series had quite a few B Stories going on...
Buffy & Angel
Buffy & Riley
Willow & Oz
Willow & Tara
Xander & Cordelia
Xander & Anya
There were more but we'll look at these to get the idea of how B Stories can put some spice into your story.
First of all, Xander's relationships with both Cordelia and Anya had little effect on the series. They were included in the series for comic effect. That is a valid reason to include a B Story...you might simply want something to focus on to provide light-hearted relief from your main story, or give the readers a breather or change of pace.
Willow's relationship with Tara served a story purpose. Tara and Willow were Wiccans and when they combined their power, they could cast spells that moved the story forward...by providing information (such as the fact that Buffy and Faith had switched bodies in Season 4), or by creating magical items that were needed to resolve C Stories. (such as the magical device that switched Buffy and Faith back to their own bodies). Willow's relationship with Tara also provided conflict in Season 4 when Oz returned and tried to get back with Willow.
Willow's relationship with Oz provided conflict when Willow and Xander had an affair and when Willow discovered that Oz was a werewolf. Oz's wolfish nature caused him to have an affair with a female werewolf, which became the focus of a C Story. The emotional fallout from these events carried over through several episodes.
Telling these secondary B Stories has a useful purpose. It reveals character. It allows you, as a writer, to explore the characters you have created and show the reader how they act in certain situations. It lets you get some strong emotion into your story.
They also allow you to create continuity through the stories in your series. If a reationship begins in story two and continues to story six, it ties your series together.
When your B Story relationships cause major conflict for the main character and entwine with the A Story, you have hit a vein of storytelling gold.
Buffy had a relationship with Riley, who was a member of a clandestine military group called The Initiative. This caused some conflict but unfortunately, Riley was boring and corny and didn't really add anything to the series. And he could never compete with the main relationship of the series...the relationship that lit a fire under the A Story in Season Three and continued to sparl conflict in Season Four and even ran into a spinoff series....
Buffy and Angel
Buffy meets Angel in series one...in the very first episode...and it isn't long before she falls in love with the dark, brooding vampire with gelled hair. This relationship causes conflict after conflict throughout three series, especially when Angel's gypsy curse is lifted and he loses his soul, reverting to the remorseless killer Angelus. This causes the death of Jenny Calendar and sparks even more conflict.
When you think up your B Story relationships, see if you can get them to kick off conflicts in your series. You'll be glad you did.
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