I'm having a problem with several actors repeatedly breaking out laughing in the middle of scenes. I had it chalked up to nervousness and was counting on it working itself out as such things usually do, but it's getting pretty late in the process. I'm running out of ideas about how to approach this. (These are adults.)

Part of the problem may be coming from that they're trying too hard NOT to laugh. My director in college had a saying, "If I say, 'don't think about elephants', what are you going to think about? Elephants." So, when someone sits there trying NOT to laugh, it just makes it harder to not laugh.

They obviously are not focused on their character or objective in the scene. I would be getting ticked off. It may be funny to the actors, but sloppy rehearsals will lead to a sloppy play. Time to crack the whip.

As easy as it is to judge and say the actors aren't focused (which may be true), that might not necessarily be the case.  I've done enough acting that usually my concentration is solid (and I'm always substantially better in performance, even when I'm having trouble during rehearsals), but sometimes even good actors have problems.  I distinctly recall a situation a few years ago, where I was appearing in The Underpants as Theo Maske.

For those unfamiliar with the play, it's a great little comedy by Steve Martin about this German housewife and her boorish husband (me) and a couple of other crazy characters.  In contrast to some of the other characters in the play, my character is still funny but is kind of the straight man and has one-liners that are a bit drier than the rest.  Anyway, there's this one scene where his wife is out and he's trying to seduce the neighbor, and I had to say, deadpan, "I am going to take off my pants."  Well, it's a ridiculous line, and I couldn't keep a straight face.  So the director wisely gathered the whole cast together and we chanted the line for a solid couple of minutes until the humor had completely worn off.  Never had any more trouble with it.  Delivered it fine during the shows and kept character, and the audience loved it.

Which is to say, sometimes even good actors are amused much more than they should be.  If it appears that a certain prompt is causing their lack of concentration, you need to deal with it a bit more creatively (as above) than if it appears to simply be general lack of focus.

And to finish the story, everyone came through with flying colors and great concentration and we had an excellent opening night last night....