Here's a MASSIVE Sonic 3 interview, featuring scriptwriters Pat Casey and Josh Miller, as well as co-producer Tyson Hesse. This was recorded before the film's release so minimal spoilers.
I really suggest watching the whole thing, there's plenty of cool info about the production of these movies. I compiled some of the highlights under the cut:
- It was SEGA's request to make Movie Sonic an alien. This is a remnant from when they were commited to the "Two Worlds" idea, before they changed their mind on that.
- According to Pat Casey and Josh Miller, part of the reason that the Sony version of the movie fell apart is that they (along with Jeff Fowler, Tim Miller and Neil Moritz) wanted Sonic to be the main character, while the studio pushed for the human actor to be the star.
- There was an outline where Sonic befriended a kid (based on E.T). Tim Miller thought the character was pointless, since Sonic is already the kid, so they decided to pair him up with an adult instead.
- They refer to "The Little Mermaid", "Superman" and "Hellboy" as inspiration for Sonic's story in the first movie.
- The Master Emerald and the Chaos Emeralds were combined to avoid having eight different macguffins in one film, and also to avoid comparisons with "Avengers: Infinity War".
- Pat Casey and Josh Miller feel like part of the job is to NOT be lore experts, but rather people who can look at these things purely as a movie.
- Jim Carrey doesn't like to repeat himself, so they always try to throw new stuff at him every movie so he's more likely to return. This led them to the idea of having him play Gerald. The studio immediately loved the idea, and so did Jim.
- One of the things they knew had to change was the ARK, as the idea of a space colony orbiting Earth for 50 years without anyone noticing didn't mesh well with the grounded world they had set up.
- In some versions Gerald Robotnik was alive as a chaos energy ghost, stuck in-between time.
- They felt Shadow's backstory was extremely important, but knew they could only have so many flashbacks, hence why they wanted to pull aspects of it into the present (such as Gerald).
- Gerald's inclusion was also done to keep Shadow's story from feeling like a retread of Knuckles'. It also helps that while Knuckles' conflict is based on a misunderstanding, Shadow's isn't.
- Tyson Hesse thinks that while keeping Gerald alive at first felt weird to him as a long-time fan, it gave Ivo a lot more to chew on as a character and kept his story from getting stale.
- Jim Carrey's multiple comments regarding Robotnik's broken childhood and hidden vulnerabilities inspired them to explore that side of the character in Sonic 3.
- Pat Casey and Josh Miller's always like to add a little bit of heart where they can. They point to the baseball scene and the bucket list in the first movie, as well the scene between Agent Stone and Tails in 3.
-Jim Carrey first does his scenes exactly as scripted, then tries his own versions, which almost always end up being funnier. Very little of what's in the script actually stays.
- Keanu Reeves was the number one choice for Shadow. With every other character there was a lot of deliberation, but with Shadow they don't think other options were even considered.
- At one point Idris Elba was worried about Knuckles' fear of ghosts being out of character, so they did some research and found out it was already a thing. They can't remember if they got that character trait from somewhere or if it was coincidental.
- Sonic 3 and the Knuckles series were worked on at the same time. Pat Casey and Josh Miller weren't involved, as they were busy with the movie script. Tyson Hesse was involved during the initial stages of the show, leading the story department, but had to leave as soon as production on 3 started.
- They've kept almost the exact same creative team thorough all three movies, including their VFX Supervisor and Animation Director. This helped streamline process, as everyone is already familiar with each other and how things work.
- They point out how rare it is to have a franchise where everyone involved wants to keep coming back. They attribute this to the fact that these movies are entirely staffed by nice people who get along, which isn't too common.
- They claim that the Sonic 3 set was one of the calmest sets they've ever been in. Even all the Gerald and Ivo stuff, which seemed so complicated to make, was hardly an issue.
- With Sonic 2 they ran into some problems due to relying on a single VFX vendor. For Sonic 3 they ran the movie as if they were the VFX vendor, they had all the animators in-house and had all the character assets made internally.
- Tyson Hesse claims that SEGA was completely changed by the movies, and that everything coming out is being done better than it was before. He hopes fans will be able to appreciate how much the movies lifted up the franchise.
