In the episode where Eric accidentally runs over Donna's cat, he's looking for advice from his family as to how to deal with it, and Red suggests just coming out with it.
"Oh, come on! How hard can this be? You go over there and you say, 'Hey! I'm a cat-killer; I mur- (laughs) (then, muffled; still laughing:) I murdered your cat...! (stops laughing) But you try to say it with a straight face."
The other family looks on horrified while Red laughs and then recovers.
Now, every time I see this episode, it seems to me like Kurtwood Smith is really cracking up and just recovers and ad-libs in character and goes with it, but the others never break and that suggests to me that they knew he was going to break - especially given that the cast corpses a bunch.
Has anybody ever said anything about this? If it's not him really busting up, Smith does a PHENOMENAL job of cracking up involuntarily; that's hard to do! The nuances he always brings to the character are fantastic. The fact that he didn't get nominated and awarded for this role is a crime.