I see this is a lot in drafts, my own included. I've also sadly seen it in published works. In my own work, I can usually spot these glimpses into the background by an extreme lack of tension.
Mary falls into the water. The cold wraps around her, and she's strangled by heavy, wet skirts. She's drowning. Is this how it ends, she thinks?
There's no struggle. No panic. She's not even trying to fight it...because in the next scene, a handsome male hero is going to dredge her from the water.
The author knows this. She wrote the story. But Mary doesn't know that. Personally, drowning is one of the most horrifying deaths I can imagine. Mary, not knowing she's going to be saved, should rightfully be terrified.
Mary falls into the water. The cold wraps around her, and she's strangled by heavy, wet skirts. She claws at the ocean, trying to reach the surface and only sinks faster. She hadn't taken a deep enough breath before she fell. Her lungs are at the limit of bursting. She pinches her lips together and kicks frantically toward the patch of light over her head. I can't die, she thinks, not now.
I think the difference is clear. In the first example, we're not really worried for Mary. In the second, we are at the edge of our seats, wondering what is going to happen.
As I'm re-reading my second drafts, I look for these spots that feel a little flat, and I stop and ask myself a few questions.
What does Mary know at this point? Does she know there's someone around to save her? Does she think she's all alone and no one saw her fall?
Once that's been determined. I ask...
Then how would Mary feel at this moment?
I guarantee if a moment in your story feels slow or boring, it's likely because there isn't enough emotion in it. And readers can see that.