Me I gotta tell you, we had a LOT of fun making the two GBC games, best time in my career (followed by the worst time in my career with the GBA Azkaban...) The publisher didn't really pay attention to our games, they were only concerned with the console and GBA versions, so we were able to do almost whatever we wanted without interference. Every once in a while we'd show them progress, they'd say "We didn't think the GBC could do this, keep up the good work". Because of that, the GBC games were closer to the books than any of the other ports, where they weren't allowed to put it in if it wasn't in the movies.
We used a bunch of tricks in Chamber of Secrets, and maxed out the largest GBC cart size, with only a few bytes to spare. The most-used trick was to do things while the screen is being drawn - load a whole new set of color palettes partway down the screen to get hundreds of colors per screen, scroll the screen at different rates on different scanlines, change the memory address of the backgrounds so we could flicker a ghostly background in front of the normal one (used a lot in card attacks). We also had a lot of background animations done by manipulating the palette, known as "palette cycling".
Ah yes, the horrible Boomslang Skin game-breaking bug. A couple hours into the game, Professor Snape has you collect some ingredients, and one is the Boomslang Skin. Unfortunately, when you get that task, if you talk to the school nurse before collecting the skin, you cannot collect the skin, and if you save, that'll save the bad state. And of course when you die you appear next to the nurse, so you might talk to her. The Japanese localization testers found that and figured out that talking to the nurse did it, too late for the North American release and the first European release (there was a second run of the game in Europe, that got the bug fixed, I don't remember if the US ever got that second run).
I never played these games, but I still love to hear the developer stories (especially the fun experiences!) I might have to put these on the to-do list if they're that good.
Easily the most underrated movie based game imho
While I never played the Harry Potter games, I thought the latter two LoTR games on GBA were pretty good movie adaptations too - they were competently done ARPGs that I liked a lot back in the day.