What is it about fiction that some people find so hard?
At its most basic, fiction is storytelling with words. Characters think, say, or "do" things that make your story move. Environment, emotion, and activity blend to make paint that forms word pictures. When all this matches up, a story flows. It can almost play like a movie in your mind.
In all cases (fiction or non-fiction), it doesn't happen if you don't write it. Failure to fill in details dooms any good stories before the first chapter is completed. There's a lot you know about your characters and situations that seems unimportant at the time you are writing. I've been guilty of that more than a few times. That's why character development matters. The men and women who save the day in your fictional world are more than champions of justice or instruments of revenge.
Speaking is more than dialogue for any hero or villain. What they say is not always what they really think or want. Sometimes, they lie. Physicality plays a role in making anyone look good, capable, or smart when they succeed. Likewise, Bad Guys who are "good" at what they do are...formidable. Before they can beg or borrow, any character needs to be somewhere before they go somewhere. Saying something in narration about their environment and atmosphere does more than set a mood, those little details help the reader see what you're talking about...in their own mind.
A dark room is more than a place without light, it can be filled with clues or crowded with henchmen waiting to do harm to unsuspecting heroes. When writing in first-person, try to remember that the "I" in your story needs to see, hear, taste, touch, and smell. If they don't, the reader has no reason to know that the place is or was on fire. Sudden surprises like that are more common than you think, we're all guilty of forgetting small things--like--who started that fire.
First-person or third-person, everyone has a point of view. All those other characters in your story who are not the hero or heroine should say something--anything--when they're not busy doing stuff. Out-of-sight is not always out-of-mind, those pesky villains are always up to some darned thing. Especially when it can spell defeat for the heroes. Violence in its many forms is just as physical as anything else characters do. Protagonists in a dark room who fight it out with concealed henchmen should move around, even if it means running away.