A common technique, at least in science-fiction and fantasy, is to include the inner thoughts of the Point Of View (POV) character. Often this is done by including the thoughts in italics without any attribution. I'm a fan of this technique, and use it in my writing. Done well, it can add flavour to a passage and deepen the immersion for the reader. One of my favourite authors, Ilona Andrews,¹ is superb at this. Take the following passage from Magic Slays:

I dropped to the ground, crawled to her, grabbed her arm, and pulled with everything I had. She screamed, but slid a foot toward me across the pockmarked asphalt flooded with melting snow. I backed up and pulled again. Another scream, another foot to the door.
Breathe, pull, slide.
Breathe, pull, slide.
Door.
I pushed her inside, slammed the door shut, and barred it. It was a good door, metal, reinforced, with a four-inch bar. It would hold. It had to.

In the above, the situation is tense — at least one person is dead, another possibly bleeding out. In moments like these, people don't often monologue to themselves, they tend to focus on the task at hand. In this case, the seven, simple, italicised words convey this and build tension.
The following is from the same chapter as above — here italics aren't used:

Andrea sighed. “It’s not good to deny the PAD access. They don’t like that.”
“Tell me something I don’t know.” Like where she’d been these past two months. Maybe she’d joined a nunnery. Or the French Foreign Legion.

This second passage gives us insight into what isn't said, fundamentally shifting the focus of the conversation. It's simple and effective. Finally, it can be particuarly fun when your character is snarky...

A careful knock echoed through the door.
Grendel surged to his feet and snarled, bouncing up and down.
It was probably the PAD come to shut me down. Knock, knock, let us in, we brought a court order and a howitzer . . . “Come in!”

Over the past year, however, I've noticed more occasions where internal thoughts break my immersion. The reason? The passages in question didn't feel like thoughts. In many cases they felt like Bond villain monologues, explaining back-story or plot points as if aware the reader existed. They were long and structured in situations where it didn't feel like the character's thoughts would flow that way.
I'm not going to include any real examples of this issue, as I don't want to single out any author or book. Instead, I've constructed a staged example that illustrates the phenomenon:

Barry handed me the manilla folder, the address scrawled on the cover.
326 Archibald Lane.
I shuddered. Not the Kim residence, they have a pool. Ever since my sister drowned in a freak synchronised swimming accident six years ago, I haven't been able to go near them. Barry knows this, why would he give me this job? I can't afford to turn it down; I still haven't paid the overdue power bill stuck to the fridge.

Most characters typically wouldn't think all of the above. It feels forced — the character knows why they can't go near pools, so they're unlikely to explain it in their head. A more natural flow might be:

Barry handed me the manilla folder, the address scrawled on the cover.
326 Archibald Lane.
I shuddered. Not the Kim residence, they have a pool. The bastard had a satisfied smile as I grasped the work order. He knows. The sod knows I can't turn it down.

The second version doesn't provide as much background, but there there are likely plenty of later opportunities to explain (or better yet, show) why pools are a problem and why the protagonist can't turn down the job. Even better, this passage now presents the reader with questions they will likely want answered, keeping them reading.
I've noticed the issue tends to crop up most towards the beginning of a novel. It feels like authors see it as a shortcut to build their world, the plot, and the characters. This can be tempting, but risks pulling the reader out of the character's head, interfering with the flow of the story. The take-away? Be true to the character's voice and the context of the situation.