The Sonic the Hedgehog movie will likely be most remembered in history for the fact that it ended up needing to create two different versions of Sonic the Hedgehog for the screen, after the first design was loudly rejected by fans. However, it turns out there were even more versions of the blue blur behind the scenes, as the production needed three different ways for James Marsden to interact with Sonic in order to make the CGI character come across as real on the screen.
Ben Schwartz, the voice of Sonic the Hedgehog, didn't need to be on set to play his role, but he was familiar with what happened on set, and explained to Polygon that three different methods were used to create Sonic, all in an effort to make sure James Marsden's eye line was set correctly, so that it would look like he really was talking to a bipedal blue hedgehog. According to Schwartz...
We had three different versions of the doll. One that I guess you’d call a doll, one that you would hold. And then for James also they had like a tennis ball. The whole thing that James and Jeff Fowler the director who's been amazing, was eye lines. They didn’t want it to look like James was just looking at a person. They wanted to know exactly where Sonic’s eyes would be and stuff like that, which I thought was so great. So there was like three different versions of Sonic.
The old "tennis ball on a stick" is the traditional method of giving an actor something to focus on when interacting with something that is an entirely digital creation, and the Sonic the Hedgehog movie certainly did that. However, they also had actual physical recreations of Sonic, or at least the general shape of the character, for the times when James Marsdan had to actually hold onto the character.
The fact that the Sonic the Hedgehog movie worked so meticulously to make sure that James Marsdan was always looking in the right place actually makes the visual redesign of the Sonic character that much more interesting. Because everything was done so precisely, the production would have been limited in the way that the new Sonic character could be designed. His eyes would have to be in the same place as they were on the previous model and everything else that changed would have to work around that.