The little dog had a shiny black coat and thick fur. It always was happy and wagging its tail. It would go run around outside and come back in panting happily.
The dog had a greasy black coat with tangled, unorganized fur. Its tail wagged too hard and always knocked things over. It never seemed to stop panting and drooling all over the house.
The diction of these two paragraphs gave different moods, even though they were written about the same dog. The first paragraph uses a positive diction and the second paragraph uses a more negative one. It is important to use different kinds of diction to convey the mood of a text
In most contexts, they are interchangeable. The only difference that most native speakers can agree upon is that someone is more formal than somebody (just as anyone is more formal than anybody, and everyone is more formal than everybody).
