Harry Potter Actors' Biggest On-Set Regret: Not Filming This Book Scene
The Harry Potter films by and large were a faithful adaptation, or at least as faithful as could be expected, but, as any sufficiently dedicated fan is aware, a number of scenes and details from the books never made it to the screen.
Of course, cramming everything from a decently-sized book into a two-hour film would have been impossible, so people mostly do not begrudge filmmakers for skipping the less important parts of the narrative.
"Mostly", because sometimes the filmmakers' choices of things to be omitted are questionable. For example, the Harry Potter and the Chamber of Secrets film omitted Nearly Headless Nick's deathday party.
As is obvious from the books, but much less so from the movies, the halls of Hogwarts are haunted by numerous ghosts: Bloody Baron, Helena Ravenclaw, Moaning Myrtle, Professor Binns, the Fat Friar, Nearly Headless Nick. And the more sociable among those ghosts, like Nearly Headless Nick, the resident ghost of Gryffindor, are considered important residents of the school, to the point that days of their death are celebrated.
While Nick is only a minor supporting character, the events which followed his deathday party in Chamber of Secrets helped to set up the main conflict of the novel, and the party itself helped to emphasize fantastic elements and weirdness of the wizarding world.
So, the cast regretted that this scene did not make the cut. The lead star Daniel Radcliffe explained in Reddit AMA that he and his fellow teen actors were surprised by its absence from the adaptation.
"I think I remember the thing early on that me, Emma [Watson] and Rupert [Grint] were all slightly surprised by, that we had all ABSOLUTELY LOVED the chapter in the Chamber of Secrets book about the – I think was it Nearly Headless Nick's Birthday Party?" Radcliffe recalled when answering questions in a Reddit AMA. "But I think it was some sort of a ghost birthday party that was not in the films, and I remember at the time we were all quite surprised by that."
As it seems, the creators of the Harry Potter film series just did not care much about ghosts. For example, Peeves the poltergeist, a notorious mischief-maker in the novels, was also not included in the films.