In the Harry Potter books, we're told that becoming an Animagus is extremely difficult to pull off, but the story never has time to go into any detail about how a witch or wizard can actually transform themselves into an animal. And it's just as well, as Harry, Ron and Hermione never actually travel down that road during their adventures. Our exposure to the ability comes from supporting characters who've managed to pull it off. However, J.K. Rowling has released a detailed rundown of exactly how a witch or wizard can transfigure themselves into an animal form, and it's now pretty clear why most witches and wizards don't attempt it often.
James Potter, Sirius Black and Peter Pettigrew could do it. Minerva McGonigall could do it. Heck, even (spoiler alert!) Rita Skeeter could do it. But Harry, Ron and Hermione never got around to turning themselves into animals. It would've been cool, and pretty convenient for them to have the ability, what with all of their stealthy endeavors. The transfiguration spell wasn't high on their list of priorities, and after reading through the instructions on how to do it, we kind of know why.
This information was apparently previously released in a 2014 U.K. special edition of the Harry Potter books, and has now become available in one of Pottermore's recently released eBooks. We're not going to go into all of the specifics on the process, but there are some key portions of the instructions outlined in J.K. Rowling's essay that really emphasize just how delicate and potentially lengthy the ordeal of becoming an animagus is. What exactly did the marauders have to go through in order to transform themselves into animals?
To start, the wizard has to hold a mandrake leaf in their mouth for a full month. As in, they can't take it out to eat or sleep or anything. If it comes out of their mouth or slips down the hatch, they have to start over.
Then, at the next (visible) full moon, the wizard must put their spit-soaked leaf in a phial within range of the moon's pure rays. If the moon's hiding behind clouds or something where it's not directly visible, they have to start over with another month of mandrake leaf sucking.
In addition to the moon's light, the potion needs some other ingredients, including dew that hasn't been exposed to sunlight or human feet for a full seven days. I'm thinking this is something a wizard will want to scout out before they start this whole process. Because if you can't find good, non-stepped on dew, guess what? You're probably starting over.
If the wizard manages to get this far into the process, they need to keep their potion hidden and out of sunlight. Sunlight is VERY bad for the potion. After that, they need to wait. Possibly a while.
What's the wizard waiting for? An electrical storm dictates the next step, as the potion can't be taken until storm lightning strikes. Every day that they're waiting for the storm, at sunrise and sunset they have to say. "Amato Animo Animato Animagus." This can go on for days or weeks or months.
That's the bulk of the process, in terms of what makes it so difficult to pull off. The rest involves preparing to transform, drinking the potion and repeating that incantation.
J.K. Rowling's detailed instructions include warnings not to panic, and also some pretty terrifying after-effects to animal spells gone wrong. There's more on that in the Animagus essay, which is well worth a read. She goes into some more specifics in terms of ingredients, what the potion looks like, and what the witch or wizard will experience during the actual process of transfiguring into an animal. Meanwhile, Pottermore has collected some other information about the process and the rules, which you can read here.
So, why didn't Harry, Ron and Hermione attempt this spell? We knew it was complicated, but the instructions really drive home just how tricky becoming an Animagus is. Especially when we take into account the weather. If the full moon's not visible, or there's no electrical storm happening anytime soon, there could be delays, or major problems. Taking the marauders into consideration, it's pretty reckless that three Hogwarts students attempted to pull it off without telling anyone, but James and Sirius were known for taking risks, so it's no surprise that they did it anyway.
And when we consider how badly things went for Hermione when she, Ron and Harry first attempted the polyjuice potion in Chamber of Secrets, perhaps it's for the best that they never attempted to transform into animals. Unlike Hermione's disastrous polyjuice experience, owing to a mixup with Millicent Bulstrode's cat hair, from what Rowling says, there's no reversing animagus mutations when they occur. So a witch or wizard who just couldn't wait for that lightning storm, forgot to do the incantation one morning, or accidentally let sunlight into their potion could end up stuck in some horrible half-human/half-rhinoceros form for the rest of their days.
If Pottermore ever decides to add an Animagus test to the site, much in the way they've done sorting and the recent Patronus test, I kind of hope they draw out the process, and really make us work for it. It seems like the kind of thing that should be set up for true fans with the determination to make it through the process, even if it takes weeks or months to achieve. The actual reveal might be less exciting in this case, for those of us who know what our Patronus is, as it sounds like the form an Animagus takes is usually the same as whatever their Patronus is.
For more details and information on becoming an Animagus, pick up Short Stories from Hogwarts of Power, Politics and Pesky Poltergeists. The Kindle Single is available on Amazon for $2.99.)