The lead character, Phil (played by Bill Murray), is caught in a time loop for most of the plot, in which he re-lives a single day many times: Groundhog day, a winter day in a small town, on which a yearly event happens (the eponymous groundhog predicts the weather), and about which Phil reports for a TV station. The beginning of the movie introduces Phil and a couple of other characters and sets the stage by running us through the events of Groundhog Day, beginning with the greeting by a pair of radio show hosts over the ether and ending with the crew of main characters stranded in the town for the night because it's cut off from the rest of the country by a blizzard.
Now, up to the moment when the day begins for the second time (indicated by an alarm clock that switches to 6:00 am and kicks off that same radio show announcing Groundhog Day), this could be a typical comedy without anything out of the normal. That we get into fantasy stuff such as time loops, reality repeating itself, is something we realize only then, that is, about twenty minutes into the action, when it dawns on both Phil and us that there is a systematic Déjà vu going on here.
What we, as the audience, suddenly have to do is accept that the world of the story into which we've got ourselves involved includes such strange elements as time loops — if not as a rule, then at least as a possibility. We have to perform what theorists call 'suspension of disbelief' on that element.
But the effect that I have in mind is not simply that we notice some fictionality indicator and must suspend disbelief: that we have to do always, with any work of fiction. (If we didn't, we wouldn't be able even to recognize it as fiction.) It's rather that we have to suspend disbelief more deeply, or in other respects, than we expected when we began to immerse ourselves, when we entered into the fictional contract for the first time, so to speak. Also, it doesn't depend on how strongly the fictional world differs from ours. It doesn't even have to be far-fetched fantasy stuff such as time loops. (One of the strongest instances I remember in a movie is that scene in Woody Allen's Annie Hall when his character draws Marshall McLuhan from behind a poster.) The important thing is that the way in which unreality comes in is unexpected, or unexpectedly intense — just as a hot shower sometimes can be hotter we anticipate. We recognize we have to adjust to some higher extravagance in this plot now than what we've been prepared for, just as we realize we have to adjust to a higher temperature than originally expected.
So much for the hot shower; let's proceed to the cold one.
Once Phil understands the new rules of his world, he quickly becomes expert at using them to his advantage. He gathers detail knowledge about everything within the restricted circle in which he can move and act, and he realizes that there aren't any consequences to any of his actions that reach further than the next morning (not even his own death, which gets reversed at six o'clock each time just as everything else). Being a rather self-centered person, he sets out to put every selfish goal he can think of into practice. (Such as stealing money without getting caught, or tricking women into one-night stands using fake common history, shared interests, and marriage promises.)
As we watch Phil gain manipulative control over the world in which he's moving, we feel that same world becoming less 'realistic', in a way which is difficult to describe: it begins to look predictable, easy to control, no longer interesting, not a challenge any more. No challenge, of course, only as long as we see it from Phil's point of view — for all the other characters, the world becomes less predictable and controllable, up to the point where, seen with their eyes, unexplainable things happen.
But of course that is the reason why it looks less like reality as we know it. In our world, we share a reality with other people, and there is no asymmetry between some who can bend the rules and those who can't, no asymmetry between people in manipulative control such as Phil and others, the clueless victims of their actions. To be sure, reality places us in situations that vastly differ from person to person, but these still are differences within reality, not within two fundamentally different sorts of reality (one which repeats itself to the point of being predictable and controllable, and one which is even less predictable and controllable than usual). The world is not a puppet play (or video game) for a single person. To the extent that the world of the movie becomes one, it becomes less like the real world, and thus less complex and less interesting with it.
To put it differently, it's at least an interesting thought experiment to imagine yourself in the main character's situation, but it's not interesting at all, let alone attractive, to imagine yourself in the position of any of the other characters. This fictional world is a very one-sidedly interesting one. It only looks fascinating through the eyes of one of its inhabitants, making all the others even more into bystanders than usual (even for a movie). The same holds for character development — there is none in anyone, except again the protagonist. You may say that this, in itself, is perhaps not that unusual: works of fictions (including movies) are sometimes very strictly focused on a single character and restrict any personal development to him, or her. But in these cases, it's a question of narrative focus; what we have in Groundhog Day, in contrast, is that the whole reality of the story is set up in a way in which there is no personal history except that of the central person. The whole of reality, so to speak, is constructed around him, and consequently there isn't anything substantial in it for anyone else, which makes it a much less rich kind of world than that in many other fictions.
It's this effect, that the world in a fiction begins to lose reality and looks like a setup, that I mean by the cold shower effect. It may be rather more unpleasant than the hot shower variant, but it's deeper in a sense, because it forces us more decidedly to reflect about the nature of that fictional world with which we are confronted; thus it drives us not only to aesthetic ascent (where we start comparing instances of fiction with each other, and possibly gain some appreciation for the performance of their creators), but also more decidedly to a more philosophical kind of reflection, where we begin comparing the structure and rules of that fictional world with what's similar to (or different from) those structures and rules in the real world.
 In the shower, we might also simply turn down the heat a bit; nothing like that is possible with the movie's story: we can only accept and leave, which latter option would be analogous to stepping out of the shower altogether. (Clearly, as with every analogy, there are limits to this one, too.)
 A somewhat similar situation emerges in the thriller Next, in which Nicholas Cage plays a stage magician who can actually see a few seconds into the future; at one point he uses this ability to play out, in his foreknowing mind, a lot of variations over how to insert himself in a row between Jessica Biel's character and her nasty ex-boyfriend in such a way that she would be interested in him (as it turns out, several macho approaches don't work, but when he lets himself be knocked out by the raging guy, we wins enough of her sympathy to get her engaged in a conversation).
 It regains its power the moment we realize Phil cannot have his way in any matter: he won't be seducing Andie McDowell's character, and he won't be able to save the dying old man, although towards both ends he tries everything he can come up with within his almost unlimited resources. There are things outside the powers of manipulation he has gained from his almost perfect knowledge (perfect within his small circle of influence). Not surprisingly, he realizes that he must change as a person to make a difference in those matters which really matter. At the moment of this realization, and the reinstatement of power to the external world (still the world of the movie), the film also gains decidedly in depth.