September 1st, 2015 marked a curious footnote in Harry Potter marginalia: According to the series’s elaborate timeline, rarely referenced in the books themselves, it was the day James S. Potter, Harry’s eldest son, started school at Hogwarts. It’s not an event directly written about in the books, nor one of particular importance, but their creator, J.K. Rowling, dutifully took to Twitter to announce what amounts to footnote details: that James was sorted into House Gryffindor, just like his father, to the disappointment of Teddy Lupin, Harry’s godson, apparently a Hufflepuff.

It’s not earth-shattering information that Harry’s kid would end up in the same house his father was in, and the Harry Potter series’s insistence on sorting all of its characters into four broad personality quadrants largely based on their family names has always struggled to stand up to scrutiny. Still, Rowling’s tweet prompted much garment-rending among the books’ devoted fans. Can a tweet really amount to a piece of canonical information for a book? There isn’t much harm in Rowling providing these little embellishments years after her books were published, but even idle tinkering can be a dangerous path to take, with the obvious example being the insistent tweaks wrought by George Lucas on his Star Wars series.

Rowling published Harry Potter and the Deathly Hallows in 2007 as the final entry in a seven-part series, and to emphasize the finality, its last chapter jumps forward in time (to 2017) as Harry sends his second kid off to Hogwarts and reassures him that he’ll be fine no matter what house he ends up in. Every loose end is tied up: The book’s students are older, married off, in varied jobs that suit their personalities, with kids laboriously named after fallen heroes and departed friends. Even within the novel, it reads a little like “fan fiction,” the literary subgenre produced by devotees of existing series that often imagines future adventures and romantic pairings for favored heroes.

So it’s no surprise that Rowling never quite put the saga to bed after Harry Potter came to a close. In interviews after the publication of Deathly Hallows, she pointed out the subtext implying that Professor Dumbledore was gay, although within the books, it’s merely hinted at. In Q&A sessions conducted on Twitter, she routinely drops other pieces of information, some monumental, others ridiculously inconsequential (here’s Rowling describing the location of a bookstore within her magical world). Many of these snippets have come through the Pottermore website (Rowling even wrote a Daily Prophet gossip column as Rita Skeeter). It seems to keep the books’ most devoted fanbase happy, but it also adds to a swelling database of unwritten information that couldn’t find its way into the stories she told.

After all, that’s how Rowling made her name in the first place: writing books whose richly detailed fantasy world powered her storytelling. After finishing Deathly Hallows, Rowling wrote a few other books (one under her name, three under a pseudonym), but she seems drawn back to the extensive universe she created, and to fiddling away on its sidelines. Her efforts parallel George Lucas’s work on Star Wars, a film trilogy he finished in 1983 and took a long break from before returning to it in the mid-1990s in advance of a prequel trilogy.

Though Lucas didn’t write and direct every Star Wars movie, he was at its creative center, and when he decided to remaster the films using more modern special effects for a 1997 rerelease, he threw in scenes he’d previously deleted and used CGI to pepper in more creatures in the foreground of his alien landscapes, all to deleterious, distracting effect. Even worse, once his maligned prequel trilogy was released, he returned to the original films again, swapping in newer actors to have them line up with the creative decisions he’d made more than 20 years later. The original cuts of Star Wars are now harder and harder to find, and Lucas’s general influence has been so destructive that fans greeted the 2012 acquisition of the Star Wars brand by Disney with cheers, because it meant the movies were finally out of their creator’s hands. Lucas’s initial interest in adding detail to his world snowballed to the point where he was no longer taken seriously by fans as the creator of his own work.

Though Rowling isn’t going back to cram new information into her books, her tweets and interview tidbits amount to annotations in the margins, little snips and changes that either sidetrack or highlight whatever points she was originally trying to make. As Lucas has shown, such an impulse can lead to larger changes that are detrimental to the original work. And given that much of the joy in falling in love with fictional characters comes from being able to envision new stories from them, by continuing to embellish her stories long after publication, Rowling is arguably chipping away at that imaginative freedom.

The motivation for Lucas to futz with his own films seems to have been his need to answer questions no one had really asked, in the most obvious ways possible. Mysterious figures like Boba Fett received extensive, plodding backstories, and the romantic decline and fall of the mysterious Jedi was dramatized as being the result of dull bureaucracy. Rowling’s coloring in backstory to inform her actual story is similarly unnecessary—if it really matters that Hagrid couldn’t summon a Patronus, or that Dumbledore remained celibate after a youthful affair gone bad, then presumably those details would have found their way into the writing, rather than being tacked on later.

Rowling has already begun to formally return to the Harry Potter books, writing a play called Harry Potter and the Cursed Child that will be staged in London in 2016. She insists it’s neither a sequel nor a prequel, but more adventures in the Potter world are planned, including a trilogy of films called Fantastic Beasts and Where to Find Them, scripted by Rowling herself, and set 70 years before the books. More formal additions could be on the horizon, considering how much interest Rowling still has in the world she created, but hopefully said works can stand on their own, rather than plugging into a fan-service feedback loop. The more Rowling enhances and embellishes her Harry Potter universe, the less room she leaves for readers to fill in the gaps with their own imaginations.