If I were in a room with twenty-five people and said: “Jimmy saw the dog.” What is the likelihood that the image that the twenty-five people brought up in their minds would be the same? If I instead said “Jimmy spotted a mangy brown and black dachshund favoring a rear leg and whimpering,” does this change the number of people with the same image?
It all comes down to what we as writers intend. Is the dog a distraction in the story, our hero flees for his life and sees it and worries it will give him away as he seeks cover? Or maybe, our hero sees a whining Pit Bull, running from a litter-strewn alley with its tail between its legs. This may cause our hero to be on guard for whatever may have spooked the animal in such a way.
But there is much more here than meets the eye. What does the last sentence say about a neighborhood that has pit bulls running free? What kind of neighborhood would have a litter-strewn alleyway? It raises questions about the threat that lies in the alleyway. Could it be that the dog has been beaten so many times that a passerby said boo and it ran? (Meaning no threat at all) Could it be that some other thing has turned a vicious animal into a craven? (Meaning some monstrous threat.)
It is the writer’s job to know and understand the effect of each description has on a reader. It is incumbent on the writer to provide as much as they can with each sentence to deliver the meaning we want to convey. The dog may indeed be intended as a simple distraction, maybe an obstacle that our hero must avoid in fear of giving himself away. (In which case less description is needed.) But as a writer, we can use that dog to convey much more. Does our hero stop to pet it? Does he kick it out of the way? In either case, we have learned something about our character.
What if the dog had been a well-groomed afghan, trailing a jewel-encrusted leash? What would that convey about the neighborhood? Had the dog been stolen and managed to break free? If our hero looked down the alley would they see a well-dressed woman in a shin-length white fur, and high heels attempting to catch up?
Remember this if nothing else: Every sentence included in a work is the conscious choice of the writer. Give those sentences some thought.