A/N: By the way, the disclaimer: all of these are based off of the Harry Potter series (which should be obvious) and the concepts for the magic, as well as the character of Minerva McGonagall, all belong to JKRowling. I make no profit, so there is not suit. Thank you.
Chapter 1: Animagi Basic Theory
The Animagus transformation is a form of wandless transfiguration. It comes from the same base magic as all transfiguration, and falls under the category of magic which changes the way things are experienced: transfiguration causes an object to look, smell, feel, sound, and even taste, different. Transfiguration does not truly make an object into something else, which is why it is reversed, and not undone. Take some of the simplest of transfigurations for example. Most would say that they turned a matchstick into a needle. This is incorrect. The transfigured matchstick is just that. It looks like a needle. If you poke your finger with it, you will bleed. It smells and tastes of steel. But it is still a matchstick. In most cases, if the "needle" is run through a fire, it will alight.
The Animagus transformation is different than other transfigurations because it uses no channel. All other transfigurations are made through the wand, even those upon the self. The power for the animagus transformation never leaves the body. In many ways, the channel for the transformation is the body.
Animagi are very rare, and for good reason. The very simple truth is that some witches and wizards will never be able to make the transformation, and that most should not try because it is too dangerous with their abilities. That said, all of the Animagi that I have met and interviewed can be divided into two categories: they either made their transformations using meticulous control over their powers, or had incredible amounts of raw power. I myself belong to the former category, and find it to be very fitting. The manner in which the transformation is brought on has almost complete control over the manner in which the transformed animagus acts. Those who approach the transformation with little or no control, and simple raw power, are in danger of losing the human capabilities of higher level thought. Simply put, control over the transformation allows for control over the creature form.
Thusly, those who transform as I did, after many attempts, and very careful control over their own powers, will maintain the same careful control over their animal forms. I have found that those who maintained this control are thusly more 'human' in their animal forms. One wizard, whose form was a gorilla, was actually able to write with a quill in his animal form, given that it had opposable thumbs. This is not always a benefit. Being to human-like in animal form usually effectively limits the form's ability as a disguise. I was once given away because, as a colleague told me, he had "never seen a cat sit so stiffly.
Those who obtain their transformations by use of massive raw power do so at the expense of control over their human functions, but are much more like the animals they become. Miss Helga Handlebroom, who has permitted me to use her name, is one such animagus. Her animagus form is a rather sleek-looking bloodhound, and because she uses raw power in her transformations, she is able to use the dog's sense of smell to a great extent. Unfortunately, while in this form, she is very easily distracted by other dogs, not to mention fire hydrants. Another effect caused by the use of raw power is accidental transformation. Many avid quidditch fans know of a very interesting Chudley Cannons game in which their seeker's broom was snapped in half by a bludger, but on the way down the man transformed into a pelican, catching the golden snitch in his over-sized mouth. This man was so far gone, that it took several trained healers from St. Mungo's to remind him that he was a man, and not a pelican, once they had returned him to human form. Naturally, he was very careful about transforming in the future.