Like all art-forms, writing has rules. And like all rules about art, they are meant to be broken.
First of all, if you’re a beginning writer, follow the rules. You must learn them before you can break them. Rules are meant to end in a certain effect. Once you have mastered them, you can see how to get the same effect by breaking the rules. Really, that’s what the rules are all about: making your writing communicate to the reader what it was meant to communicate. Not just to make a sentence that isn’t confusing, although that’s important too, but most of all to convey the emotion and general ambiance of the scene, give a sense of the setting, and help the pieces flow together as a whole.
So it’s about the end result, about making a certain effect. For instance, I’ve known writers who wanted to declare that a run-on sentence is one that extends for more than two lines. (I mean, first of all, line length are partially determined by fonts and sizes, so that’s hardly consistent.) But the problem with run-on sentences is not that they extend for more than two lines; the problem is that they drag on with too many ideas and make the brain run out of breath. If your sentence doesn’t do that, it’s all good. I’d dare to say that it’s not a run-on sentence at all. I’ve seen writing where the author’s sentences naturally rolled one into the other and rolled me along with it, making the actions smooth and fluid instead of halting.
Along that note, novels are admittedly not formal papers. In real life, people talk and feel and think differently than “the rules” might dictate. Your writing should reflect that in a realistic manner. Going strictly by the rules can lead to writing that’s too rigid. Just don’t be excessive.
Second of all, you must never break the rules just because it’s easier that way. (Is that a rule of writing? And if it is, does that mean it can broken? Hmm.) You can usually tell a beginning writer by the way their writing is just floundering around in punctuation, sentence construction, and wording that is awkward and incorrect. There’s a palpable difference between the greenhorn writer who doesn’t know what their doing and the seasoned writer who’s purposely bending the rules to their advantage.
I just finished reading the science-fiction classic Dune by Frank Herbert and it’s a perfect example of what I mean. He constructs sentences incorrectly, but you can tell it’s not because he’s ignorant or lazy; you can tell he really means it. And because it’s his natural style flowing intentionally out of him, it makes a spell-binding and unmistakable effect. For example: He reached into his tunic, brought out a sheath with a black-ridged handle protruding from it. (From Dune by Frank Herbert.) Hopefully you noticed that the sentence is incorrect, grammatically speaking. Yet the result is a more right-now perception to the action. To me it feels like one action follows from another the way it would in real life. At the same time, I wouldn’t endorse everyone writing this way. And he pretty much did that every time there was a sentence like that, and I’m not sure that was entirely a good thing either. But all that aside, it marks an individual style and lends a particular, deliberate feel to the writing. It felt authentic.
And that’s what the rules are all about: writing that is authentic, natural, and easy to read. So follow them. Break them. But write well.