You start a new paragraph when the subject changes, and if you're writing dialogue, also when the speaker changes.
It depends on what you're writing. Nonfiction writing seldom uses dialogue unless it includes quotations. In fiction, however, dialogue is almost essential to a good story. People talk. It's hard to have a good story when nobody is talking.
Starting a new paragraph when writing dialogue helps to clearly indicate a change in speaker, making it easier for the reader to follow the conversation. It also helps to separate the dialogue from the rest of the narrative, enhancing readability and flow.
Tom Chiarella has written: 'Writing dialogue' -- subject(s): Authorship, Dialogue, Technique, Fiction
a dialogue is a conversation in speech or in writing between two or more persons. The word also means the words spoken by actors in a drama.
If you are writing a nonfiction work, you do not need dialogue. A fiction work needs dialogue to advance the plot and make it more interesting.
The poems are of course poetic, and so is most of the dialogue in the plays, but not all. Some of the dialogue in the plays is written in ordinary prose.
It depends on what sort of book you're writing. There can be dialogue if it's a fiction book. A nonfiction book usually does not contain dialogue.
instructed, advised, informed
Each time a different character is speaking
informal
A brief pause in the dialogue