I wonder how popular the Muppets were when Walter first knew of them. In the flashback, he first knew of the Muppets upon renting a Muppet video as a child, after being somewhat older than a baby. I would think he would have known about them by that point. But then again, the kids do laugh at him for wearing a Kermit costume... It'd be weird that he first learned of the Muppets after they'd been out of the public eye, and became a big fan.
I think somebody mentioned that Rizzo was credited among Steve Whitmire's characters. I wonder if he really did. The director said he wanted to give Rizzo lines but couldn't work them in, implying there's no deleted scene with him, and when Steve Whitmire was interviewed for Tough Pigs, which occured when production on the film was ending (if not already complete) he said that he hadn't performed Rizzo in a few years, even in productions Rizzo appeared in. So I doubt Whitmire performed him here. The only scene he would have been able to perform Rizzo in would be in the finale when Rizzo and the rats appear with Beauregard. Then again, we know that they reshot the ending, I can't remember if that was after the interview.
I've read that Tyler Bunch performed Thog in all of his scenes, even though the character didn't get any lines. I wonder if Matt Vogel performed Sweetums in every scene he appeared, whether he spoke or not. I don't think any of Vogel's other characters had lines whenever Sweetums appeared on-screen.