Period. Full Stop.

The teachers who drilled the rules of punctuation into students -- commas mean a pause, period a full stop -- were not fiction writers. It's not that black and white. The music of the writing comes from words in varying combinations and also punctuation. A particular narrative voice has its own rhythm. Stacatto sometimes.
Short sentences. Simple declarative sentences without embellishing clauses. Sometimes one word sentences (but shouldn't it be a good, useful, even powerful word?) alongside which a pedant might be inclined to scrawl in red, "inc.!" for incomplete sentence.

If it takes too long to establish a voice, the reader will be confused. Will struggle to catch it. Broken punctuation rules will look like mistakes. Readers won't be able to trust their "guide" - as Alistair MacLeod once called the narrative voice - through the story.

Ali Smith is an unconventional punctuator. Lots of semi-colons, colons, irregular line spacing, at least in the first few pages of her How to be Both. For example,

                                                                                    "eyes  :
                                                                                        hello  :
                                                                                          what's this?
                                                                                                A boy in front of a painting.
Good  : I like a good back  :"

I have been resisting, but maybe I will try again. Maybe the resistance has come with an unconscious desire to cleave to the voice that speaks in my head every time I sit down to work on revisions to my own novel.

Long sentences make another kind of narrative voice and require very careful punctuation. The work of some writers - Faulkner, MacLeod - fall into this category. Faulkner created a parade of description and action and reflection all in one sentence, often with a minimum of punctuation. MacLeod's sentences roll like the sea, and the commas he includes serve as troughs between the waves, a sort of rocking rhythm. From The Boat (1968). "He sang all the sea chanteys that had come across from the old world and by which men like him had pulled ropes for generations, and he sang the East Coast sea songs that celebrated the sealing vessels of Northumberland Strait and the long liners of the Grand Banks, and of Anticosti, Sable Island, Grand Manan, Boston Harbor, Nantucket and Block Island. Gradually he shifted to the seemingly unending Gaelic drinking songs with their twenty or more verses and inevitable refrains, and the men in the shanties smiled at the coarseness of some of the verses and at the thought that the singer's immediate audience did not know what they were applauding nor recording to take back to staid old Boston."

A thought occurs in time. Em dashes, hyphens, commas, semi colons, colons and even periods all qualify as pauses. Some long, sometimes glancingly brief, some representing afterthoughts.  Parentheses are good at this last job. Exclamation points are noisy. Question marks? A response not always necessary. Backwards apostrophes can help create a dialect, and right-ways apostrophes, too, at the other end of the word. Most readers won't notice how deliberately some writers work to achieve a distinct voice.  To other writers it comes as a gift and expresses itself almost as if impossible to do so in any other manner. It doesn't matter as long as the voice becomes sufficiently fixed in the writer's head that s/he can transpose it into the reader's imagination, individual, fully realized, irresistible.