Bruno reminds me of Mr. Bean. They obviously don't resemble eachother, but their small words and grunts are pretty ingenius. His demeanor kinda gives off a bit of grumpiness, similar to Oscar. Maybe that's why they mesh so well.
I remember him being quiet most of the time, but I wouldnb't have held it agaisnt him if he had grunted or such. In Big Bird's Birthday, right before Bruno starts skating, oscar asks if he's ready, and I feel like he should have made a "Hmm" sound. He didn't, but should have.
And it is interesting during that skating sequence when Oscar says "We'll talk about this later". I wodner if Oscar regularly said that to Bruno. There isn't much Bruno material in fan circulation, but that would be funny, Oscar saying that they'll talk about somethign later, when bruno rarely spoke.
But I figure that the effect used to move Bruno's mouth must have been limited, since when he does talk, it's usually just two one-syllibal words. In The Trash Can he does get a longer line ("put it in the trash can"), but he's not on-screen for most of that line (instead, it's a shot of the audience).
Though an invisible wire was used to move his mouth, I wodner if it was possible for Caroll Spinney to put his hand inside the mouth whenever Bruno wasn't carrying Oscar or any other characters (I recall a scene where he carried a can with Irvien inside). I watched closely on the trash can song, but couldn't find the wire, though that could be the video quality on youtube. The mouth movement seems a bit like something pulled by string. I was also watching closely at the end of Follow That Bird yesterday, to see if the wire might have gone through the mouth so it would stay up, and I didn't see the wire there, either. The film quality might have made it more invisible than video quality, though, after all, the wire connected from Big Bird's hand to his chest to his other hand wasn't noticable in the movie as well. But I have seen a wire connected from Bruno's mouth, in a few photos (like the one in Sesame Street Unpaved, which is one of the photos on his Muppet Wiki page) and in Sesame Street, Special, wher ehe doesn't even open his mouth.