From Writers Helping Writers:
Cause and effect. Stimulus and response. Action and reaction. Everything in a story depends on what the characters do about whatever the story pushes them up against.
Stiff, disconnected, or missing character reactions snap the chain of cause and effect that constitutes your story. When readers can no longer see how and why the characters are doing what they’re doing, they lose the thread.
Let’s talk about the three most common action–reaction misfires I see in manuscripts.
1. Missing or insufficient reactions
2. Jumbled responses
3. Purposely obscured stimuli
Missing or Insufficient Reactions
When characters fail to react to what’s happening around them, it’s as if nothing is happening at all. A snappy line of dialogue goes nowhere if it doesn’t get under someone’s skin. The first glimpse of a long-sought clue builds no excitement if nobody notices it. A punch in the nose might as well not have landed if it doesn’t start or end a disagreement.
When characters don’t react to the conversations and events around them, readers will assume they don’t care. If the characters don’t care, why should readers?
Keeping your characters engaged in the story keeps readers engaged with it too. When writing viewpoint characters, you have access to both internal and external responses. For other characters, you’re limited to whatever visible manifestations of those responses that the viewpoint character or narrator can perceive.
Internal Responses
All but the last type of internal response, thought, are involuntary reactions.
1. Involuntary sensations—These include physical sensations such as feeling a lump in the throat or a stomach full of butterflies.
2. Reflex reactions—These are the so-called knee-jerk reactions, such as jerking away from the source of pain.
3. Emotions—Before you can reveal emotions using any of these reaction modes, you as the writer must know what the emotion or blend of emotions actually is.
4. Thoughts—What’s the uncensored commentary running in the privacy of the character’s mind?
Link to the rest at Writers Helping Writers