Unlike Sonic 3, I don't chuckle

User muteKi posted over at the Sonic Retro forums:

One of the rather defining bits of Sonic 3 is the many moments where Knuckles comes around to throw a curveball into your path through a stage, with a distinctive giggle animation. (...) Lava Reef, however? (...) He just stands there, like a schmuck. No giggle there.

In none of the S&K cutscenes does he giggle, despite it being such a distinctive animation in Sonic 3.

Knuckles isn't playable in standalone Sonic 3, so there's no reason to include all of his sprites in the Sonic 3 cartridge, especially since player graphics are uncompressed and thus take up a lot of space. Instead, the game only stores the sprites used in each of his cutscene appearances, including the trademark giggle.

In Sonic & Knuckles, however, the game has access to nearly all of these poses from Knuckles' main art block, so the cutscene block isn't included. As such, the giggle animation fell by the wayside.

The interesting bit is that in the earliest prototypes we have, he does giggle in Lava Reef.

In the April and May prototypes, the LRZ2 scene uses the cutscene art block, along with the punching animation from Launch Base 2. Starting from the 0606 prototype, it instead uses the player art block and a custom animation. Rather unsurprisingly, the 0606 prototype is also the first build which no longer includes the Sonic 3 half of the game.

My guess is, when it came time to remove all the Sonic 3 dependencies, it didn't make sense to keep the cutscene art block around for just one level, and it was too late in development to do something like split off the giggling sprites and their mappings, and rewrite the object so it switches art and mappings mid-execution.