Regardless of the genre, the introductory paragraph always explains what the rest of the piece will be about. It must include details that the reader needs to know regarding setting and characters. The length of the piece determines the amount of exposition that the writer needs to include in their introduction regarding the specific topic. If you are writing a piece that is based on mystery and suspense, you have the option of revealing as little as possible to draw the reader deeper into the story and reveal pertinent details in later paragraphs or chapters.
Academic writing, from five-paragraph essays to doctoral dissertations, must contain a thesis statement. This is one sentence that is integrated into the introductory paragraph that explains the purpose of the paper and the argument the writer is trying to make. Fictional writing does not require a thesis statement in the introduction. Writers of fiction use writing tools like dialogue and similes to reveal what motivates the story in a more subtle way.
Journalists and writers of fiction can use narrative devices like allegory, foreshadowing and the first person voice to make their introductory paragraphs more effective. Journalists often write in the first person for feature articles to help familiarize a place that may be exotic to the reader. News reports are often written with shorter sentences to convey facts and sound bites more quickly. For writers of fiction, publishers often use the first paragraph to judge the merits of a book and determine whether or not it should be published.
All writing has structure, but some genres are more organized than others. Academic writing has the most strict outline, most of which is based on the five-paragraph essay structure, which requires that an introductory paragraph include an explanation of the structure of the piece along with the thesis statement and brief explanation of the subject matter. The structure includes the arguments that will be presented, in what order and how the author has come to the conclusions presented. Fiction follows a plot chart that has many possible variations, but the even the introductory paragraph of a very long narrative requires setting the scene to some degree.