I thought I would be the biggest Harry Potter fan in the theater. I was wrong.

On Saturday night, I lucked into a ticket for the second half of the new, hot Broadway show: “Harry Potter …

On Saturday night, I lucked into a ticket for the second half of the new, hot Broadway show: “Harry Potter and the Cursed Child,” Parts One and Two, a sequel to the magic school series phenomenon, now in previews at The Lyric Theatre. You can see the two parts separately or on the same day, adding up to some five hours of thespian adventure.

Though I missed the first part, I was able to quickly orient myself because I had devoured the special “rehearsal script” shortly after it was published in 2016. I thought that preparation plus years of Talmudic devotion to the seven original books, affiliated movies and fanfiction put me in a high percentile for Harry Potter-fandom among the theatergoers.

I was wrong.

Prepare for the Potter superfans to establish a long and excited residence on 43rd Street.

By way of example, here is how the waning minutes of intermission played out in the balcony during Saturday’s performance of Part 2. A haggard usher, who looked like he had grown a full beard over the course of the evening, walked down my aisle transfigured into a literal embodiment of exasperation, anger and despair.

“We will not be able to start the show until whoever took a child’s booster seat returns it,” he explained.

That’s right: Someone was so set upon seeing the (admittedly extremely impressive) theater magic, that they snagged a kid’s extra padding and hid it, invisibility cloak possibly utilized.

The next act started so apparently the culprit relented, but clearly someone was really on the edge of his or her seat.

Then there was the way the show ended. I’m not talking about the script’s final words from Harry Potter, wrapping up the show’s and series’ major themes with a bow. No: Saturday’s rendition ended as the actor playing Harry was still in the middle of making his final motions (I won’t spoil them or various other plot points because I don’t want to get doxxed by superfans for the rest of my Muggle life). A woman a few seats away from me started screaming. Not applauding loudly or cheering but screaming, like she’d been bit by a basilisk or was wandlessly facing down a Hungarian Horntail. The kind of screaming that sounds like there’s an emergency, which is how Harry reacted, poor soul.

He jerked backward, putting a hand up in front of his face, as if He Who Must Not Be Named was about to appear. When Harry saw the noise was just someone very eager to applaud, he relaxed. Just another day’s work as the Boy Who Lived.

That was what happened just within a few broomsticks’ distance from me, so you can imagine what will take place theater-wide, day in and day out as “Cursed Child” opens in full on Broadway later this month. To glimpse the wider picture, I went back to haunt the theater, reached out to people who had seen or auditioned for the show, and also preserved my own memories in a Pensieve to be able to deliver them to you all today.

This magical quest revealed that the superfans were around from the beginning — they showed up to audition in Midtown as early as 1 a.m. on the day the proverbial sorting hat would make its choices. Some people hoping to enter the Potter universe came from other states and left before even getting seen, so thick was the crowd, says one tipster. Among the crush: one big dude already in costume as Hagrid, apparently just in case.

At the theater, things have often been wild just west of Times Square. Lines extended down the block for tickets like Ollivander had opened up a new outlet, and the ticket scalpers swarmed in the manner of the dementors ably portrayed within.

Inside, it’s not rare to see theatergoers dressed for the occasion, and not just children throwing elbows to get autographs by the stage door either.

Alexis Poindexter-Jenkins, a 25-year-old Brooklynite who saw the play on Sunday, says she noted a very well-dressed 30-something couple draw wands and duel on the way to the bathroom.

Poindexter-Jenkins also noted the surprising appearance of an adolescent boy seated near her who seemed to be embarrassed to be there, particularly after his mother asked theater staff about the possibility of getting a frosé and a booster seat (the booster seats!). That was at the end of Part 1. But by the end of Part 2, after watching the show itself, “he was thanking his mom for bringing him and was so happy,” writes Poindexter-Jenkins in an email.

Mischief managed. The magic continues.

AMNY Newsletter

Eat it. Drink it. Do it. Tackle the city, with our help.