The “Whatever you do don’t laugh” trick can work on a single person in my experience. Again, start off by telling them to do a serious face first. You also gotta draw it out a bit when you’re telling them not to laugh, but say it in kind of an exaggerated way that’ll be more likely to cause them to break.
Two things I use that have worked fairly well for me:
-Get the group all together, and just before you start shooting say, “OK now pretend like you all like each other.” Usually gets a fairly genuine laugh/smile from everyone.
-Have two people stand next to each other, tell them to make a serious face while looking directly at each other. Then all you gotta do is repeat, “OK now whatever you do, don’t laugh. DO NOT LAUGH.” That combination is almost always guaranteed to make them smile and/or laugh genuinely.
You’re an idiot