Thanks to language classes everywhere, most of us know the basic rules about creating paragraphs. A lot of us suffered through rules mostly for essay writing, but it's actually very helpful in other formats!
If you need a brush up, check out the Purdue Online Writing Lab and search for "On Paragraphs." That site will give you a great idea on what a paragraph is and what to keep in mind as you write.
Of course, the focus there is on essays and more formal writing. In fiction we've got characters, actions, plots, descriptions, dialogue, and more that can also influence our paragraph breaks. Our paragraphs aren't often proper paragraphs, but for the sake of ease that's what I'm calling them today.
So, if you're struggling a bit with deciding what should or shouldn't make up a paragraph in fiction, here's a bit of help!
Change in scene.
This one is rather obvious. If you're changing scenes, take a paragraph break. This lets you find the scene, your reader find the scene, and it gives everyone a chance to process what happened and prep for something new.
New speaker=new paragraph
Keep readers aware that there has been a shift in perspective/character. A new speaker gets a fresh line to minimize confusion over who said what. In addition to providing clarity, it breaks up your page by adding white space, which is friendly and inviting to readers vs a wall of text.
For example:
I drummed the edge of the warm sheet. "So what do you think?"
My brother took a bite of the cookie. He frowned. "Eh, it's okay," he said and reached for another.
There are times and places where you could probably get away with two people speaking in one paragraph, but there's also a chance you could probably get away with picking your nose in front of your crush. You really don't want to do it unless you know things'll work in your favor. And I'm telling you right now, they probably won't.
New speaker, new paragraph doesn't always mean new character, new paragraph.
When you're writing dialogue, you don't want to have two characters speaking in the same paragraph (unless it's one of those odd "Bob and Bill said...."). However, if your characters are performing actions, they don't always need to be separated.
For example:
The egg rolled across the tilted counter. Jake lunged, missed. At the last second, as if on instinct, as if I cared about our stupid home-ec 'baby', my hands extended. The egg plopped into my cupped palms.
Here we have three actors: the egg, Jake, and our narrator. Our story would start to look a little silly if every time we changed characters, we had a new paragraph. White space is inviting, but too much of it and you're going to be writing a list, not a story.
This is where other understandings of paragraphs come in. What we grouped together above is cohesive in the sense that it all revolves around an action and the immediate resolution (the egg falls --> I catch it).
So let's go to the next elements:
Give something its own paragraph for pause and emphasis. Combine something into one paragraph for (generally) speedier action. Remember though, long paragraphs read slower than shorter ones.
IE: paragraphs aid in creating tone and pace.
In the above example, the egg's fall was a bang-bang play. The narrator (and by extension, the reader) has hardly any time to react. Everything is happening at once, and quickly.
Periods, commas, semi-colons, and line-breaks all serve to give the reader a pause (the same way a Chapter often acts as a stopping point for a moment or a lunch break). The length of that pause varies depending on what you used. You can use that timing to your advantage, as I talked about in the chapter on basic pacing.
Group for cohesion.
It could be a theme or a set of actions. Maybe a description or an idea or a fleeting thought. We've all seen these paragraphs. Open your favorite story and you'll find them in droves. One paragraph details the monster, another the sunrise. Maybe one paragraph is dedicated to a summary of the MC's travels from the crime scene (current scene) to his hotel room (where the next relevant scene takes place).
For example:
Jackson Makebelieve was everything Mariah imagined in a man: tall, handsome, and currently holding in his possession a very thick wallet. Tapping the leather bi-fold with a touch of impatience, he sat wedged between his companions like a lone, sober wolf among cocks. Where his mouth remained pursed in dogged weariness, his friends hooted and clawed at the performers' heels.
"Dalia. Dalia!"
Mariah turned her head at the sound of her stage-name.
A good way to start grouping is to ask yourself what the point of the paragraph is. Here, the point is to introduce Jackson. We draw attention to him by setting him apart from his friends. This is just an example, so I'm not going to invent three or four more sentences to go with it, but you get the point.
Story within a story? Time flashing back or forward? Use a break to begin.
Again, this just cuts down on confusion for the reader, and the paragraph is an excellent way to help signal a time shift (but obviously, don't just rely on a paragraph to transition from the modern day to the past; you'll need other cues).
Break when you've gone on too long. [also a sign you may need to edit]
It is very hard to stay focused on massive walls of text with no breaks. If you find yourself needing a break when you're writing or reading it over, there's a chance your reader needed one five sentences ago. Maybe someone's more invested in your story than you are, but there's many more people reading than that one shining star. So give your average readers a break. You'll likely instinctively know when you've gone on too long. A simple break can bring a lot of refreshment if that's the case!
For example:
I don't want to type a long example here, but think about when another character is telling a story about something, say, how the magical unicorn horn came into their possession. Their story is one cohesive unit, but if it's very long, you might need to break it up to relieve the reader's eyes. One or two paragraphs. Where you choose to break it depends on what you "point" is. Maybe the first paragraph is about what the unicorn horn is and what they were doing when they found it.
And the second paragraph talks about what the character is now doing because they have a unicorn horn stashed in their backpack. It's up to you! Sometimes it can be obvious; other times you may just need to find that spot where you read out loud and have to pause for a breath!
HOW LONG ARE PARAGRAPHS IN FICTION?
About 5-6 sentences if you're excluding those one-liners, zingers, stunners, etc, but really they can be any length. Let's say we're just talking about an average figure. Depending on the genre and age group you're writing for, you might find that 3-4 sentences or 7-9 is more appropriate. You might find that word count also plays a role.
7 sentences * avg. 11 words per sentence = 77 words
7 sentences*avg. 15 words per sentence=105 words
BUT THERE IS ONE IMPORTANT THING TO NOTE:
PARAGRAPHS (and their sentences) SHOULD BE VARIED.
This helps pacing. This helps catch and keep your reader. The varied, unpredictable shift from 3 sentences to one to seven and down to four is what keeps the page interesting to look at and read. It's harder for the eye (and reader) to get bored with all that movement. Your paragraphs should not all be 9 sentences long. Have one with 80 words, another with 130.
As the story progresses you will start to see what your average looks like, and can visually see the paragraphs that go very far above and below that number.