How to make a flat sentence interesting
Rebuild this sentence in your own words before you read how I would.
The old house was very creepy and mysterious.
The usual fix for a flat sentence is to decorate it: add an adjective, upgrade a word with the thesaurus, bolt on a simile. The result is a flat sentence in better clothes. The real problem is almost never the vocabulary.
The sentence
The old house was very creepy and mysterious.
A composite — built from a pattern that appears on many students' pages, not quoted from any one of them.
The diagnosis
The sentence delivers a verdict — creepy, mysterious — without calling a single witness. Creepy is a conclusion the reader is supposed to reach, not a thing the reader can see; handed over as a label, it asks the reader to take the writer's word for it. Readers never take the writer's word for it. And very is the tell: intensifiers cluster around words that aren't carrying weight, the way scaffolding clusters around walls that can't stand alone.
Three moves
Move one — arrest the label, demand the evidence. Ask the sentence: what did you see that made you say creepy? There is always an answer behind the label. A gate that has rusted open. Curtains in a window nobody has lived behind for years. The answer is the sentence.
The gate had rusted open, and the curtains in the upstairs window were still drawn.
Move two — pick one detail and go closer. Two details is a list; one detail, examined, is an experience. The curtains are the better witness — they imply a person, and implied people are what makes a house creepy rather than merely empty.
The curtains in the upstairs window were still drawn, as if someone inside had a reason to keep them that way.
Move three — let the reader reach the verdict. Delete every trace of creepy and mysterious and check the sentence still produces the feeling. If it does, the labels were redundant. If it doesn't, the evidence wasn't good enough yet — go back to move one.
The after
Nobody had lived there for years, but the curtains in the upstairs window were still drawn.
No adjectives of atmosphere at all — and more atmosphere than the original, because the reader has been given the two facts that don't sit together and left to feel the gap between them.
The principle
Specificity is interest. A flat sentence is almost always a general one: it reports the category (old house, creepy) instead of the instance (drawn curtains, rusted gate). The revision that works is not upward, into fancier words, but downward, into particular things — because a reader can only feel what they can picture, and nobody has ever pictured very mysterious.