Getting into the characters’ heads is one of the most important things you can do for your reader. It’s especially important in horror where it’s always necessary to portray psychological terror through this method. This is what authors like Stephen King do masterfully. Also, think about Lovecraft and how his characters experience terror.

So, let’s first define interiority so you can use it in your own writing later on:

A character’s interiority refers basically to two things:

Thoughts and emotions + Narrator’s/authorial comments.

Thoughts portrayed directly or indirectly make up half the character’s psychological makeup. These include ideas, perceptions, recollection, memory, past actions, remembrance, imagination, logical reasoning, creative brainstorming, hunches, guesses, interpretations of actions and events, and all other mental operations.

Emotions are reactions to events or thoughts. The emotional reaction shot is paramount in writing good, realistic characters. Every time something relevant happens in your story, you should explore an appropriate emotional reaction. Showing and telling both play a role here. In writing good fiction, emotional reactions are usually shown using body language.

Let’s say your belt breaks and your pants drop while presenting your latest product to a round of investors. What’s your reaction? You’ll be embarrassed, for sure. That’s the emotion. Embarrassment. But what’s your body language, physiological reaction, and rational physical reaction?
You blush, sweat, pull your pants up, and then apologize.
And if you are lucky, your audience will laugh but won’t kick you out of the meeting. 

So, let’s sum up:

Interiority is made of thoughts, subjectivity, interpretation of events, and emotions. When writing fiction, thoughts are all mental operations, and emotions are all physical/physiological reactions—mind and body.

Emotional reactions are part of a character’s interiority. Okay, but what else could I link to interiority while trying to get into the heads of my characters like Stephen King does?

Well, authorial comments or indirect free discourse. Remember that interiority includes all mental operations, including those of the character and the author/narrator.

Free indirect discourse blends the perspectives of the author-narrator and the character. It provides a way to shift between these two perspectives, between character interiority and authorial comments, and also balances internal life and external events. FID is a powerful tool that immerses the reader into the mind of the character.

So once you wrote your character’s interiority, you can switch to authorial comments, keeping the character’s subjectivity and idioms to add an extra layer of interiority, this time, coming from you, the author, and being your interiority and responses to the events taking place in the story but using your character’s voice, language and idioms. It’s like the narrator-author becomes infested by the character’s voice, idioms, and perspective and writes his comments from both the character’s and authorial worldview.

It’s like author Tim Waggoner says in his “Horror is a state of mind” article from Crystal Lake’s publishing “Where Nightmares Come From”:

“The horror should come first. Your story should grow from the twisted seed of horror, and the best way to make this happen is to understand that horror arises from consciousness. Horror is an emotion, a reaction to something that violates our sense of what we believe to be our normal and (mostly) safe reality. In other words, horror begins inside a character’s head. The story isn’t about what happens. It’s about what a character perceives to be happening, and how that perception impacts the character.”

So, if you want to write absorbing narratives, remember to open up your character’s mind to your reader. This is done by including interiority. Interiority is made of everything that goes through your character’s mind plus your own authorial comments using your character’s voice and idioms. And if you can add your emotional reaction shot using body language to complement interiority, you’ll be well on your way to write like the authors you love.

This is all for now. Best of luck.