Chekhov's Gun, for the uninitiated, is an allegedly literary technique named for its formulator, playwright and author Anton Chekhov, who opined, in varying ways at varying times, that an author should not describe a gun up on the mantel in the first act unless somebody gets shot with it in the third act.
Stripping away the metaphor: don't put any detail in the story if it doesn't move the plot forward.
Rowling was cited as a Gunner because things that were tossed in as throwaway mentions would later turn out to be significant later on. But her books were full of detail... she didn't just put a gun on the mantel... she put a porcelain doll and a music box and the family coat of arms and a Chia pet up there, and then she waited three or four books before bringing one of them into play. She was very good at having tiny little seeds grow into surprising (but entirely plausible in retrospect) trees, but she was hardly following Chekhov's advice.
I think I made a blog post at one point (I really should tag my entries better) on the subject of how a reverse formulation would be better advice to writers: don't have somebody pull a gun out of their ass in act three if you need to shoot somebody; show the gun in act one. That's what Rowling does. She shows her guns. In some cases it's likely that she showed a particular gun knowing exactly when it would go off, how, and why... but it's easy to imagine her looking back at the rich array of ornaments she'd hung on the wall and selecting the one that best fit the needs of the evolving plot.
And what's wrong with that? Nothing. Chekhov's advice makes sense if you're writing a certain species of tight, tense, terse drama. Following it slavishly reduces all literature to that one narrow genre, though. Rowling crafted a world that wasn't afraid to spiral and sprawl, wherein each book had a thousand different hooks that could (and did) lodge themselves in the mind of a reader, and thereby captured the imagination of a generation of readers in a way she never could have if she'd stripped the story down to "Harry kills Voldemort" or even "Harry confronts Voldemort and learns lessons about love and death" with no extraneous details.
So let's not suggest that Rowling was a follower of Chekhov. She was breaking the rules and she was doing so magnificently. That's why the next time somebody tells me I need to brush up on Chekhov's Gun... or worse, accuses me of being a proponent of the technique (it's happened)... I'm going to cite Draco's Wand as an alternative principle:
"Fill your story with enough details to keep the reader's imagination going, and yours will never fail you."
We know that Rowling had certain details of the series planned out, but we also know from her own words that she changed things as she went, that the story evolved and sometimes went in surprising directions. But because she'd taken the time to spin out all these background details, she always had material to work with... instead of one measly gun on the mantelpiece, she always had a weapon at hand when she needed one.
None of this is to denigrate Chekhov (my beef with him is more of a "protect me from your followers" thing) completely, because there does have to be a balance between the main plot on one side and detours, waysides, and background details on the other... but to point out that his is not the only approach, nor the most valid for every form of writing. Slavish obedience to his maxim not only results in boring stories, it can dumb down the state of the art of storytelling.
Rowling's runaway success is largely the result of her runaway style of storytelling, and this will hopefully prove to do much to counter those who bitterly cling to their Gun like it's a religion. There are places for brevity and simplicity, and there are places for expansive details.