Like in 85 and Biolantte. You could see his upper lip move to bare his teeth. Like a dog would do. After those movies that disapeared. In 85 after he blasted the hole in the building. He did it. Then in Biolantte, after the Rose form grabbed his leg tripping him. When he got back up he did it. Man I just loved seeing that. Lets you know he was getting ticked. After these 2 movies they didn't do it again.
Another was in Mechagodzilla 2. When he finished off MechaG in the first match. You could see his nostiles flare just before his victory roar. Now that was neat. Another they didn't use again.
I think they had animatronics in the heads of some of the suits. In G vs MGII there were motorized "brow ridges" so he could frown and narrow his eyes and stuff. I thought that was pretty cool. I also liked the big ol' sneer. Makes me wonder if he can grin at ya too XD