The problem with any unwritten law is that you don't know where to go to erase it. -Glaser and Way
There are always rules. Don't head hop. Don't use fragments. Don't use run-ons. Show, don't tell.
Elmore Leonard would have you believe you should simply "Try to leave out the part that readers tend to skip."
Well now, that's easy, isn't it? All you have to do is know exactly what every one of your readers likes to read.
But is it really that simple?
Take the Arwen half of Marilu Mann. She loves descriptive passages. Give her lengthy, detailed narratives where every dress is described down to the stitching on the hemline and she's happy. The Cai half of Marilu Mann would rather have her eyes gouged out. She prefers witty, scintillating dialogue and action to propel her story forward.
A well-known "set-in-concrete-don't-you-even-think-about-breaking-it" rule is no head hopping. Basically this means that you should stay in one character's POV (point of view) for at least the full scene. I can easily rattle off several big name authors who trample that rule like it was a small bug in their way as they head to the bank to cash that next six figure royalty check. There are some who stick rigidly to that "rule" with every book they write, but most don't.
And for goodness sake! Don't. Use. Fragments. Right. I'm betting Faulkner could be used as an example of taking that rule a bit too far. Fragments actually work well in action scenes. The sparing use of them can help punctuate better than the ever-dreaded exclamation point. (Arwen cringes here because she knows Cai is looking at her with that look!!!!!) What? (Arwen LOVES exclamation points - AND CAPITALIZATION for emphasis!!!)
The "show, don't tell" is a good one for the most part but there are times when you just need to tell. Sometimes it is okay to say, "the man sat down in the chair" rather than "the man strode into the room as if he owned the place. Eyeing the furniture disdainfully, he hooked a chair towards him with his foot, then straddled it." Hmm. Here is a case of Arwen preferring one over the other. Cai thinks it depends entirely on what comes before and what comes after...sometimes less is definitely more, at others you can draw out the tension in a scene by expanding it in subtle ways.
So what does that mean for you, the writer?
This all comes back to another "set-in-concrete-don't-you-even-think-about-breaking-it" rule. That rule is TELL A GOOD STORY. This is one that really never should be broken. When you're concentrating all your efforts on telling a good story and the reader recognizes that, they probably won't notice that you've disregarded a few rules along the way.
The true take-away from this? Just keep the rules in mind as guidelines rather than boundaries. See them as fences to help corral your wayward thoughts and words but don't worry if you go free-ranging every now and again. Pushing the limits can be a good thing when done with intention.
And when you can't help but break some of those unwritten codes, remember the immortal words of Captain Barbosa from Pirates of the Carribean, The Curse of the Black Pearl "...the code is more what you'd call "guidelines" than actual rules."