Jim Carrey remarked during his press promotion for the film that "very little of the dialogue from the pages of the script ended up in the movie". He also was given a lot of creative freedom for his non-verbal scenes, receiving basic instructions like "Here's the room and this is the music, just do something to go with it".
When the first teaser poster was released, Naoto Ohshima, the character's original designer, and Yuji Naka, former head of Sonic Team, said they disliked Sonic's appearance. The first trailer caused a massive fan backlash. On May 2, 2019, director Jeff Fowler announced on Twitter that Sonic's design was going to be altered to make the character "the best he can be." The movie's scheduled release moved from Thanksgiving weekend 2019 to Valentine's Day 2020. When a new trailer and poster were released on November 12, 2019, fans and critics alike praised Sonic's new look.
What attracted Jim Carrey to this project more than anything else was that his daughter was a big fan of the Sonic the Hedgehog games growing up.
Jim Carrey entirely improvised Dr Robotnik's dance scene. Carrey also recommended the song "Where Evil Grows", which he had heard as a child.
Jeff Fowler explains the Sonic references as being put in generally throughout filming: "It's not like you're going out of your way in terms of the story to put these things in. They are what they are, which is just a way of rewarding fans for their years of enthusiasm and getting to see some things that they would recognize but some that the rest of the audience might not."
Garry Chalk: as the Navy Chief Of Staff when government and military officials discuss the power outage Sonic caused. Chalk had previously starred as Dr. Robotnik in Sonic Underground (1999), and as Grounder in Sonic der irre Igel (1993) and Sonic: Christmas Blast (1996). (This isn't a Cameo, as it only is used for *uncredited* actors and Garry Chalk is listed in the credits.)