Recently, Sonic the Hedgehog made the spinning leap from video games to movie screens. Or in most people's cases these days, onto their TV screens via video on demand. When the first trailer for the movie dropped, fans were understandably upset by the titular character's appearance.
He looked a lot different from the video games. The changes they made were not good, to say the least. Thankfully, they took the CGI back to the drawing board and redesigned Sonic to be much closer to his original style. But that wasn't the only difference between the games and the movie.
Sonic's Origins
Where this ridiculously fast blue hedgehog came from in the games isn't really discussed that much, probably because that info wasn't necessary to enjoy smashing into things as him. In the movie, Sonic was adopted by Longclaw the Owl, reputedly the last of her warrior race. She took the orphaned hedgehog under her literal and proverbial wing to raise him. When he needed to escape, Longclaw protects him, presumably dying in the process though that was never confirmed.
Sonic Is An Alien
In the context of the video games, Sonic just is what he is. Theoretically, there are other beings like him on his world. As the games progressed, other amazing anthropomorphic characters were added to the roster, such as Tails and Knuckles.
But in the movie, Sonic escaped his pursuers by jumping through a portal to Earth. The reality of this was it made Sonic definitively into an alien, at least relative to how humans would perceive him.
Sonic Has Blue Arms
This is more of a design issue than anything else. When the first horrific trailer for the movie dropped, no one could understand why Sonic looked the way he did. The CGI design was a weird "uncanny valley" attempt to make him look more human-like, a mistake the filmmakers thankfully corrected. But one of the elements to survive the redesign were Sonic's arms. In the games, his arms have generally been a pinkish skin tone with no hair. In the movie, they were covered in blue hair. It's not a huge deal but it's also one of those things fans could subconsciously know was off without realizing it.
Sonic's Motivation
The video games, particularly the early ones, had a strong environmentalist message to them. That version of Sonic loved nature and hated authority, especially anyone who would harm his home. Doctor Robotnik's plans by definition put him at odds with Sonic.
He sought total domination, and the destruction of the environment was a side effect of that. The movie loses the environmentalist aspects of Sonic's motivations, instead replacing them with pure survival and defending his new family.
Robotnik Works For The Government
While the movie version of Doctor Robotnik had his own motivations, he worked for the Government of the United States. Based on the response of various people from the government and military early on, he wasn't someone they were interested in bringing in but was viewed as a necessary evil. The video game version is beholden to no on but himself, seeking world domination and the establishment of his own empire.
Sonic Isn't A Drifter
One of the key qualities Sonic was given in the games was an almost nomadic lifestyle. He actively fought against the idea of being tied down in any capacity. Just ask Amy Rose. But the movie painted a much different picture of the hedgehog.
Sonic yearns for family and inclusion, desiring a home more than anything else. It's unlikely that the video game version of Sonic would have stayed on Earth long enough for Robotnik to have even become aware of his existence.
Sonic Becomes A Superhero
Taking that a step further, Sonic was always depicted in the video games as doing what had to be done to defend his home. In many ways, he seemed reticent about his desire to fight. If anything, he just didn't seem to like Robotnik and that was enough to go up against him. But the movie version of Sonic wants to be a hero. He's even spotted reading Flash comics, the most famous heroic speedster of all time.
Sonic Can Swim...Kind Of
As tough as Sonic is, the blue hedgehog does have his fair share of weaknesses. He's not a huge fan of Robotnik's creations nor does he like the spikes that seems to be superfluous around his world in the games. A big one for him seems to be water as Sonic can't swim.
The movie was a much different story. At one point, he ran across the country and into an ocean. He returned wet, covered in seaweed and had a fish on his head. Sonic probably still wasn't a huge fan of water but at least it didn't kill him. The fish didn't fare as well in that scene.
Robotnik's Nickname
The full name of Sonic's frequent opponent is Doctor Ivo Robotnik, also known by his nickname Doctor Eggman. In the games, that name comes from the fact that he was most frequently depicted with a bald head and an egg shaped body, though there have been some updates over the years. The movie version played by Jim Carrey was certainly not egg shaped, though a later scene does show him bald. Sonic gives him the nickname "Eggman" after noticing many of the good doctor's robots are egg shaped.
None Of The Humans Exist
This is the big one. The biggest difference between the human characters in the movie and the games is that they never existed in the games. Moving the setting from the various worlds Sonic fights his way through to Earth necessitated the inclusion of a variety of human characters who simply aren't necessary in the games. Sonic having to interact with them was the biggest difference to his character between the games and the movie.