Introspection is one of the great joys of novels. When characters let you inside of their heads, you get a different sense of who they are. You receive direct information on their motivations. In many ways, characters in novels allow themselves to be known by us more than the people around us.
However, introspection often turns out to be the most tedious part of storytelling. I think this is because writers, who have formed habits of editing in other parts of their stories, let things go in fluid, almost stream of consciousness way, when they get their characters thinking. Another common sin of writers is to use introspection as a way to toss in backstory and information readers "need to know."
And, perhaps the worst offense by writers as they create their characters reflection is paragraphs of indecision. Will I? Won't I? This sort of dithering shows up especially often in manuscripts by beginning romance writers. Something close to this also appears in genres with action, such as some science fiction, where characters get engaged in considering alternatives for their plans.
The first defense against bad introspection is understanding why it is in the story to begin with. The primary value should always be to expose aspects of the character. Readers give us permission to stretch reality so that they can get closer to our characters. Any introspection that doesn't lead to a better understanding of who this person is squanders an opportunity.
Of course, the character is revealed directly by what is valued and what is noticed. A heroine reflecting on lovemaking tells us a lot about herself by disclosing what the experience meant to her and what she considers worthy of deliberation. An important aspect of this is the emotional content. As with all scenes, a passage that includes introspection needs to have an emotional arc. This can be conveyed by the choice of words or the expression (often revealing surprising perspectives), but it's also valuable to break away from introspection to include physical responses, such as increase in heart rate.
In fact, often the best introspection is constantly being directed and redirected by action and dialogue within the scene. Making the introspection part of the general mix grounds it and keeps it from becoming overwhelming. There's a great example in this article by K.M. Weiland.
Special care needs to be taken with action scenes and love scenes.
There's a tendency to interrupt and slow things down with introspection.
Anyone who has ever been in a challenging fight knows that there's
little time for reflection and most of this is not deep. Likewise,
lovemaking is all about being present in the moment, not judging,
remembering, and questioning.
But what about scenes that consists entirely of introspection? Rarely, these can enhance a story. At a key point, the protagonist may face a difficult choice before taking action, one that test him or her and requires everything the character has.When the character is approaching a turning point that requires important changes and includes key realizations before such action can be taken, a scene that is mostly introspection can add layers of understanding.
A good way to think about constructing such a scene is to imagine it as a monologue. The best monologues are addressed toward someone else or toward a resistant aspect of the speaker. "To be or not to be" in Hamlet is a conflict-ridden argument about suicide. Thinking in terms of persuasion can help to create a scene of introspection that reads quickly.
So, for faster reads, writers need to understand why they are writing introspection, have a bias toward interspersing it with action, dialogue, and other stimuli, and trimming it of all the "realistic" meandering that distracts from the purpose. And, within all of this, the introspection must pass the main test — does it reveal character? Keeping these rules of thumb in mind will help you take full advantage of the value of introspection while helping you to avoid the pitfalls that drive readers away.