I think I just haven't totally disengaged from the last big project yet. Probably doesn't help that I'm cleaning it up a chapter at a time for the Quill version, and therefore still in regular contact with it. At any rate, doing something that's resembling a second draft has me thinking about the theme of the thing--what's it all about, Alfie?--just as a kind of focusing tool (Stephen King refers to it as "something like a magnifying glass" in the writer's toolbox in On Writing). I knew I was thinking about the relationship between the shapeshifters Remus and Tonks (hence the title), but looking at it again, I'm seeing a whole lot of shapeshifting... which isn't surprising, since I'd been noticing around the time I started it just how much shapeshifting there is in Harry Potter canon.
So what is it about shapeshifting? Why is it such a common motif?
In Campbellian analysis, the Shapeshifter is kind of a wild card--a representation of the animus or the anima, someone who "serves the dramatic function of bringing doubt and suspense into the story." I don't think that seems to apply--particularly the first part--in terms of HP shapeshifters, or shapeshifting (since not everyone who shifts is someone who does so naturally).
These are the types of shapeshifts I've noticed:
Animagi: Of course, this is the biggest category that we see in the books (for shapeshifters, not necessarily shapeshifting), despite being rare in the wizarding world in general. We have McGonagall, Sirius, James, Peter, and Rita Skeeter. The rules of this shapeshift seem to be that it can be done at will and by choice, but the person doesn't have a choice of what the animal will be, nor can the person change the animal. Like the Patronus, it's unique to the wizard or witch in question and appears to represent something of importance to him.
Werewolves: Okay, it's singular, one werewolf, but the rules appear to be the rules. This is an involuntary and uncontrollable shift, and the end result is an animal which is the absolute worst that it's possible for a person to be--an aggressively cannibalistic beast. It's the ultimate breakdown of the self, the consciousness... the total loss of autonomous identity.
Metamorphmagi: Again, only one in the books, but she's not the only one who's ever lived, so we can take her as an example. This is a totally voluntary and totally chosen ability to shift shapes. We don't know what the limits are entirely (though we never see Tonks do anything radical, like change herself into another species--or a man--we don't know that she can't; I just assume that she can't because it makes it more interesting), but there presumably are a few. Voluntary and mutable, it seems to express the whims and transient wishes of the witch or wizard involved.
Shifts via potions, plants, charms, etc: The biggest category in general--people are always charming and Transfiguring things and one another. The twins make people turn into canaries (or grow long tongues) by eating special effects sweets, Hermione's teeth grow madly (only to be reduced into something new) when she's hit with a hex, Dudley ends up with a pig's tail, Harry turns into a human fish when he chews gillyweed, a cauldron explodes in Potions and people swell up ridiculously, etc. Voldemort forces himself into the back of Quirrel's head. Can be voluntary or involuntary, and the effects of it are partly choice, partly fixed--you decide which potion, charm, etc, to use, but the results of using it are stable. The most important example of this type of shifting is the use of Polyjuice Potion in books two and four, first by the heroes, then by the villain, which allows them to slip into new identities for awhile and see other people from a different perspective. (It's arguable that the reviving potion Peter makes for Voldemort is also a shapeshifting potion, which is pretty darned important as well.)
Nonmagical shapeshifting: Being a magical environment, this is probably the least-used kind of shift, but we do see it. Viktor Krum, in Harry's perception, goes from being a godlike flyer to being duckfooted and clumsy. Hermione changes her entire appearance for the Yule Ball in GoF (yes, she uses a potion on her hair and her teeth are changed magically, but the change seems mainly to be in her bearing and in physical features which she hasn't altered from their real shape so much as worked on cosmetically). Dudley goes from being a fat lump to being a well-trained athlete. These changes, like the changes of metamorphmagi, are voluntary and chosen, though they take a lot more work and aren't as immediate.
Now, some of this shifting does work in terms of setting up plot points, but it has less to do with the shifting itself than with what people are doing with it. (Eg, Sirius is able to get on and off the grounds as a dog in PoA, which stymies people, but the fact that he's a dog as an animagus is more of a character point than a plot point. He could as easily have been Hannah Abbot's flowering shrub, creeping over the walls, to serve the plot.) What it does seem to address, in every instance, is the question of identity--who each person really is in essence. I think a shapeshifter in an existentialist world would go crazy quickly, because if there's no ultimate identity (no primary essence), then all there is is the shift. This doesn't appear to be the case in HP. Why are there so many? I think it's because the books are quite focused on Harry figuring out who he is and where he belongs--hence the focus on the father quest, which is ultimately the quest for the autonomous self (no accident that when Harry believes he's seeing James, it's actually himself that he sees).
The animagi issue is the clearest identity call--what sort of animal a person becomes is a lot of self-knowledge and understanding, which is probably why it's so difficult to achieve. It's almost a question of finding one's totem animal, something that represents a person on a spiritual plane more than an animal one (unlike lycanthropy). We "meet" Sirius in his transformed state as the Grim (a spiritual being), and of course Prongs appears only as Harry's ghost-like Patronus. There's no spirit connection to Scabbers the rat, but then, Peter rejects a higher calling to his life.
The werewolf represents the stark idea that no one--not even the up-until-then nearly saintly Lupin--is without the raving beast in there somewhere. No one is clean of it, and under the right circumstances, it will come out. How does this feed into identity? The beast appears to be what happens when the identity is ripped away. Lupin refers to the greatest benefit of Wolfsbane as its ability to help him keep his own mind during transformations, making him able to exert control over the beast, even when he can't stop himself from becoming it. The werewolf is the opposite of the animagus. The transformations, instead of concentrating the identity into a totemic shape, temporarily suspend identity and make him into a totally undifferentiated monster.
The metamorphmagus (and most of the other minor kinds of shifts) represents experimentation and discovery of the identity. Tonks is constantly shifting, apparently just for the fun of it, but when we first meet her, she's studying herself in the mirror, trying to decide whether or not her purple hair is "right." Unlike the werewolf, there appears to be nothing dangerous about her shifting; unlike the animagus, there is no ultimate conclusion to her shapeshifting, no gestalt in which she sees, "Aha! This is my totemic self!" She just keeps searching at this point in her life, trying on new faces, trying to find one that's "right." And yet, specifically because she has the ultimate range of choice, this is a person who really has to know who she is--otherwise, she, like the werewolf, could be come totally dissociated.