For a reader, tension results in a page-turning state of mind. It's wondering whether the main character will overcome the obstacle in that chapter to get to what she wants. When there is no obstacle, you risk a boring chapter. Let's talk about two scenarios I see most often where tension is lacking...
No Learning Curve
A common problem for new writers is things coming too easily for a character. Maybe she gets new powers, and instantly she's slinging fireballs or flying loops in the sky. Boring. My favorite part of any super-hero story is when they first acquire the abilities. I like seeing the emotional struggle to accept that this happened to them. I like seeing them fumble and break things. When something drastic happens to you, you don't blink and go, "Cool, I'm awesome now." Your mind reels. Reality is off-kilter. It takes a while to stop saying, "WTF is going on??" In fact, it takes more than just one chapter. You have to milk it, draw it out.
Acceptance Comes Too Easily
Let's say your main character wants to come out to her parents. "Mom, Dad, I'm gay." It's a big deal. She's appropriately apprehensive. You've got all this great tension going before the big moment. And then... "Why honey, we're so glad you told us. We support you no matter what." Wah-waahh, totally anti-climactic. All that great tension you built earlier fizzles and evaporates like water on a lava flow.
FiveArchangels made a great point by commenting, "As a member of the LGBTQ+ community, I personally like when they psych themselves out and then their parents support them, as long as the parents are those types of characters." I agree completely. It's also realistic for a kid to work themselves up into a lather, and it ends up going well! If the parents are the accepting kind, then it makes complete sense for them to be nice about the whole thing. In this case, it may feel like a relief for the reader rather than a fizzle. To work this angle, you have to build up the apprehension beforehand. Make her psych herself out.
In most other cases though, acceptance doesn't come easy. People need time to process big news, and they do so in different ways. Some retreat into stony silence. Others throw huge tantrums. Still others attempt to reason their way out of it. Denial is natural, and most people will go through it before they can reach acceptance.
A best-selling author once revealed that, during her revision process, she'll go through each chapter and rate each one on a tension scale of 1-10. If any chapter fell below a 3, she would find a way to add tension. Also, she would make sure the biggest dramatic scenes were 9 or 10. Not every scene needs to be 9 or 10. In fact, they shouldn't. Tension should ebb and flow. This makes the highs higher, the climax more dramatic.
Trying to figure out what to add to increase tension doesn't have to be hard. If you have a slow scene, things are going right for the main character. Time to throw in a monkey wrench. Think, "What would really suck for her right now?" or "What's the worst thing that could happen right about now?" Throw that in and have her deal with it, and you've got instant tension.
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