Make 'em laugh. It's all about having a merry time. Typically one or two players start to figure out what's going on ahead of another player...wink and keep the ball rolling and see how long it goes until the last guy catches on.
If a player is deadly serious about it, it won't give him what he needs and it will fall flat.
Then again, maybe he's not the Merry Christmas type, anyway.
I resemble that guy....if told ahead of time that we'll do a one-off adventure and told what to expect, I can play along and do my part. Use new characters, or we all have the same strange dream, or it's some parallel dimension - I can deal with this. Tell me we're doing a Paranoia style adventure in Araka-Kalai and we're all Ivashu, and I'll have a blast. But if I'm expecting a serious scenario that fits into the campaign and you spring something like this on me, I'll be disgruntled at the least. Don't tell me that gargun are irredeemably evil and must be destroyed, then expect me to react well when all of a sudden they are merely misunderstood and it turns out I'm the evil villain committing genocide. Don't change from a Deryni setting to He-Man cartoons in midstream. If you've been doing swashbuckling jokers with bad puns and lame quips, don't drop a grim and gritty storyline with torture, rapine, and emotional crisis in our laps without talking it over with the players first.
The campaign style is a contract between the GM and players. Some players don't care - they just want to show up, hang out, and goof off, and don't mind if a new ruleset, campaign, or genre is chosen each week. Others care very much, and need the consistency in play and campaign setting and style. For these players, you must let them know ahead of time that you plan a change, or the fun they get from it is lessened.