I write about it not because I am the master about to impart some wisdom, but because it is something that I struggle with as well. How do you add enough tension to your novel to make it interesting?
First off, a writer needs to accept the fact that tension is essential. Necessary, even. There's a reason why romance novels don't continue once we reach the "Happily Ever After" part. There's a reason that, after Harry Potter defeats Voldemort in the 7th book, there aren't another 7 books following Harry on his happy journey through life (although as a huge Harry Potter fan, I would love another 7 books about Harry). The reason is because just like conflict is necessary, books need tension to move from conflict to conflict without losing the reader's interest.
Think about your life. Think about when you meet up with your friends at the bar/coffee shop after work on Fridays. What do you talk about? The highlights of your week, sure, but also the low points. The triumphs and conflicts. And the events or feeling that occurs between those moments. That feeling that carries between and surrounds conflicts is tension.
You don't tell your friend all about how you went to work with no troubles, went about your day with no troubles, talked to your boss with no troubles, and went home to watch some unexceptional TV with--you guessed it--no troubles.
You would tell your friend that you had a good day, or an average day, or a boring day, and then skip ahead to something more exciting.
And when we write, that "more exciting" means conflict. Trouble. Danger. And the tension that comes with that.
But if you're a writer you know that stories need conflict. Anyone who has taken a high school (maybe even middle school) English class knows that stories consist of beginnings, conflicts, and resolutions (often shown by that triangle/mountain graphic. And the journey from conflict to conflict is full of tension.
I'm feeling my own tension now, with my book release tomorrow, and the poor people in my life who have to hear about the worry mixed with excitement surrounding that release ad nauseam probably wish that I would focus on the boring parts of life. But that isn't how story telling works, even in "real life." Conflict and tension. They drive the story just as much as your characters.
Which brings me to the role of characters in creating tension. Throw problems at your characters. Make life hard for them. And then, for the sake of all that is good in the world, make sure they feel the tension. In my serial novel The Bodyguard, Lakshmi lives a sort of double life. This creates problems. Conflicts. But she doesn't move seamlessly from conflict to conflict without a care in the world. She worries. She feels stress. Her emotions are on edge due to her secrets and feelings. She doesn't just face her conflicts and move on, she carries tension from one to the other.
And as a reader, we hope that Lakshmi finds some release from the tension.
As a writer who loves Lakshmi, I don't like turning her life into a tangled mess. I don't want bad things to happen to her. I want her to live in Happily Ever After world. But I don't let her, because that would be boring. So I throw in conflict, and I make sure she knows it. And that causes tension.