Scripted yes but have you ever seen a blooper reel from any comedy movie at all? The characters are enjoying the joke, and the characters told to keep a straight face, even despite knowing the script, often struggle. They did here too.

Further, I was specifically referring to the scripted element, where the emperor is (in the script) waiting for them to break.

You are a bitter person, who is shilling for a super old movie? You didn't even critically read my comment, why would anyone care about your input if your already off on all counts?