How can you increase reader tension? I mentioned that it isn’t the character that has to feel the tension but the reader. One way to achieve this is through tweaks to your main characters. Remember that surprise is key to reader tension.
One reason readers become complacent is that over time, they learn to predict character reactions. This is especially true in the middle — after character has been established. By the middle, readers think they know everything there is to know about the character. If you allow this, there is risk that the reader will get bored.
Use their assumptions against them while remaining true to the character you have created.
Give your character an unexpected reaction. Unexpected to the reader, that is. You will have to justify the reaction and the readers will see that there is more to the characters than they thought, maybe an unmet need, controversial belief, previously unrevealed back-story.
NOTE: this works best if you set up reader expectations before challenging those beliefs. Find several similar situations in your manuscript that evoke the same reaction. Then insert or alter a situation that is almost identical except that it includes a personal trigger that will cause the character to react differently. The reader will be surprised, confused, intrigued. The different reaction will put the reader on edge until the change of behavior is explained (in a way that stays true to character). Done right, you can also deepen character with this technique.
Another way to use your main character to raise reader tension is to exploit his/her character arc. A character becomes predictable when turning points are delayed for too long, lack surprise twists, or have steps that do not offer enough change in character outlook/motive/behavior.
These techniques should help you to keep your reader guessing and engaged with your main characters. Keep the reader unsure, keep the surprises coming, and you will be well on your way to raising reader tension.