Spoilers to follow, obviously.
In Fantastic Beasts and Where to Find Them, we learn that
Percival Graves is actually Gellert Grindelwald in disguise.
My assumption is that this character stole the identity of the person they are pretending to be, presumably by killing them and taking their place. I believe it would take more time than the character had (as they were busy doing other things) to construct an entirely new persona and rise to such a prominent position, and there's indication that the two characters had met in the past.
However, at the end of the film, we see
Newt removes the Graves disguise, revealing him to be Grindelwald
using the Revelio charm.
So, my question is, what method did the character use to disguise themselves?
The obvious answer would be Polyjuice Potion, but:
We never, to my recall, see this character take any drinks during the movie. Given the previous books/movies, this seems like an odd choice to make by the filmmakers if it was indeed suppose to be Polyjuice, which has to be taken every hour.
The character is unmasked using the Revelio charm. In Goblet of Fire, after Crouch Jr-as-Moody is knocked out, Dumbledore and Harry wait around as "minutes pass" for the potion to wear off. If such a thing could be undone instantly by the Revelio charm, why wouldn't Dumbledore have done it?
This would likely require keeping the real character (if there is one) alive a la Moody, and preparing constant batches of Polyjuice potion, which seems difficult and time consuming.
However, if there's another way, say, a spell, and that's what's use in Fantastic Beasts--why have we never seen it before?
I recall seven instances of people using Polyjuice potion in the books: Hermione and co in CoS, Crouch Jr. in GoF, Crouch Jr./Mrs. Crouch in the Gof backstory, the Seven Potters in DH, Harry at the wedding in DH, and the Trio breaking into the MoM/Gringotts in DH. (Please correct me if I've missed any.) Any of these people could have benefited from a quick spell rather than a nasty tasting potion.
I would consider the above especially for Mrs. Crouch and Crouch Jr. as Moody, who had to keep up this for months. What if someone walked in on them as they slept?
So, if a such spell exists, why not use it in the above cases?
Here are some things I've thought of and rejected:
It's really difficult magic, and only the extremely powerful FBaWtFT character could do it.
I would buy this for Hermione at age 12 and maybe the dying Mrs. Crouch, but not for the rest. Crouch Sr., Crouch Jr., the Order, and Hermione at 18 are all extremely powerful themselves.
It's really rare magic, and people haven't heard of it.
Then why did the characters know that the Revelio charm would do anything? If I suspected someone was in disguise in the HP world, I would wait for Polyjuice potion to wear off, rather than attempting a spell I didn't think would work. Also, Hermione spent a LOT of time researching in CoS, I'm sure she would have discovered this.
It requires killing the person you're disguising yourself as.
Possible, but given lack of evidence I'm not going to buy it unless I'm given this more explicitly.
The "real" character in Fantastic Beasts isn't dead: they were never real in the first place, so the bounds of the spell are different.
Given Graves' position in MACUSA, it seems unlikely Grindelwald could be doing his terrorizing in Europe at the same time as leading the Aurors in America, or that he could rise from nothing to that position in the time since his disappearance.
"Revelio" is such an easy spell that the main HP characters didn't want to risk discovery by it.
The character in FB wasn't scared--and just taking away someone's ability to drink liquids for an hour is pretty easy too.
This spell can only change "surface features," a la a glamour. More complicated transformations need the Polyjuice Potion.
This would make sense for Crouch Jr. pretending to be Moody, as he'd have to lose an eye and a leg. However, I don't think it stands for several of the other HP cases.
The character in FBaWtFT is a Metamorphagus or otherwise has innate powers that most wizards and witches couldn't use.
Again, I'd need more proof or indication in order to accept this; there's no basis.
If I've misstated or assumed anything, let me know!
So, given all the above: how was the character in Fantastic Beasts disguised?