"an immensely complex spell ... involving the magical concealment of a secret inside a living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is henceforth impossible to find -- unless, of course, the Secret-Keeper chooses to divulge it." (PoA pg 205/152)
J.K. Rowling gave us further information on the how the Fidelius Charm works. In a FAQ poll on her official site, she says:
When a Secret-Keeper dies, their secret dies with them, or, to put it another way, the status of their secret will remain as it was at the moment of their death. Everybody in whom they confided will continue to know the hidden information, but nobody else. (JKRowling.com)
So, although Flitwick describes it as "complex", J.K's description is seemingly straightforward, the information is concealed within the Secret-Keepers soul, and there it stays for an eternity, unless the Secret-Keeper chooses to communicate the knowledge. But is that the whole story? No.
The house at Godric's Hollow, 12 Grimmauld Place, and Shell Cottage are three properties that are protected by the Fidelius Charm, three properties that have been secured against Voldemort and his Death Eaters. But within each property we see differences in the way the Fidelius Charm works.
We are told Peter Pettigrew, by virtue of the charm cast by either Lily or James, was the Secret-Keeper for the Potter's home in Godric's Hollow. But in the final book, Deathly Hallows, J.K. establishes how and why that secret is no longer kept:
"Harry --" "Look... Look at it, Hermione ..." "I don't ... oh!" He could see it; the Fidelius Charm must have died with James and Lily. (DH 331/271)
We have been told by both Flitwick and J.K. herself directly that the Fidelius Charm has a life span irrespective of the Secret-Keeper, that "the secret dies with them". But here in Deathly Hallows is an example where that's not true, in this case, the Secret-Keeper (Pettigrew) is still alive, but the secret has, nevertheless, been revealed to the world. And J.K. tells us through Harry's narration that the reason the Fidelius Charm was broken was because the spell caster (James or Lily), NOT the Secret-Keeper, had died. Is this a mistake in the books? Maybe not.
This seemingly contradiction leads us next to the headquarters for the Order of the Phoenix, 12 Grimmauld Place, and a whole host of new questions.
We are first told by Sirius in Order of the Phoenix about the Fidelius Charm on 12 Grimmauld Place, when he tells Harry:
"Dumbledore's Secret-Keeper for the Order, you know -- nobody can find Headquarters unless he tells them personally where it is" (OotP 115/107)
Dumbledore is Secret-Keeper, but who cast the charm? I think it is generally assumed that Dumbledore himself cast the charm, but this cannot be so, because if you follow the above example the spell only breaks upon the death of the caster, we see now that the location of 12 Grimmauld Place is still protected after Dumbledore's death.
It seems a logical conclusion is that Severus Snape could have been the caster, as it has to be someone still alive after Dumbledore wasn't. This is just another example of how deeply Dumbledore trusted Snape, to have given him such a powerful position in the secret of the headquarters of the Order.
OK. So now, because of how it is described why Harry and Hermione can see the remains of the house at Godric's Hollow, we have a new understanding of the complexity of the Fidelius Charm. So, J.K.'s description of the charm should really have been:
When a Secret-Keeper dies, their secret dies with them. Everybody in whom they confided will continue to know the hidden information, but nobody else. UNTIL the spell caster dies, then the secret is revealed to everyone.
But wait, the spell gets even more complex. In Deathly Hallows, we learn why the Burrow has replaced 12 Grimmauld Place as the headquarters of the Order:
Mr. Weasley had explained that after the death of Dumbledore, their Secret-Keeper, each of the people to whom Dumbledore had confided Grimmauld Place's location had become a Secret-Keeper in turn. "And as there are around 20 of us, that greatly dilutes the power of the Fidelius Charm." (DH 90/79)
So, was it a mistake to tell so many people the secret of 12 Grimmauld Place? It would be unavoidable, everyone who was in the Order and had to go there would have to know. Does this infer that Dumbledore knew the spell would eventually be very diluted, and so therefore only intended that the building be the headquarters of the Order for a short time?
We must ammend the original description yet again:
When a Secret-Keeper dies, their secret dies with them. Everybody in whom they confided will continue to know the hidden information, but nobody else. UNLESS the Secret-Keeper dies, then all those who know the secret become Secret-Keepers. UNTIL the spell caster dies, then the secret is revealed to everyone.
In Deathly Hallows, we learn about another complexity of the charm, and why it's so tricky to use. After their escape from the Ministry, Hermione says:
"As we Disapparated, Yaxley caught hold of me ... we arrived at Grimmauld Place ... I've already taken him inside the Fidelius Charm's protection. Since Dumbledore died, we're Secret-Keepers, so I've given him the secret, haven't I?" There was no pretending; Harry was sure she was right. It was a serious blow. If Yaxley could now get inside the house, there was no way that they could return. Even now he could be bringing other Death Eaters in there by Apparition (DH 270/223)
When Hermione inadvertently revealed the secret to Yaxley, since she was a Secret-Keeper, she has given him the secret. So, as we can see now, for the Fidelius Charm to be effective, both the caster and the Secret-Keeper must remain alive, and the Secret-Keeper needs to give out the information necessary with extreme caution -- one mistake will cost both the Secret-Keeper's life and lead to the revelation of the secret.
And this uncovers another big flaw in how the Fidelius Charm works. If you know the secret of a hidden place, you can bring in someone else who doesn't know the secret, if you bring them in using side-along apparition.
So the charm has huge flaws -- but what about its complexity? How difficult a charm is it to cast, and how long does it take? How does it take effect? Does the property simply vanish into thin air, concealed as Grimmauld Place is in nothingness? Who can see it the moment the charm is done -- are you exempt if you are within the boundaries?
Trapped in the dungeon of Malfoy Mansion, Harry tells Dobby:
"Right. Dobby, I want you to grab Luna, Dean, and Mr. Ollivander, and take them -- take them to --" "Bill and Fleur's," said Ron. "Shell Cottage on the outskirts of Tinworth!" (DH 468/379)
And Dobby obviously finds Shell Cottage, as we see Luna, Dean and Mr. Ollivander had made it there. A little later, Harry and Dobby, who had returned to the Mansion, escape in the middle of the fierce battle to capture him. Harry grabs Dobby's hand and disapparates them both through the sheer power of his will to Shell Cottage.
Back at the cottage, after Harry buried Dobby, Bill tells Harry:
"...The Death Eater's know Ron's with you now, they're bound to target the family" ... "How are they protected?" asked Harry. "Fidelius Charm. Dad's Secret-Keeper. And we've done it on this cottage too; I'm Secret-Keeper here." (DH 482/390)
Dobby found Shell Cottage the first time on his own, so it couldn't have had the Fidelius charm on it at that point. It couldn't have been added in the time between Dobby's first trip and when Harry brought Dobby back there, because even if Dobby had been told the secret, he didn't have time to tell it to Harry, and Harry found Shell Cottage all on his own, so it couldn't have had the Fidelius Charm on it then either. So, these new Fidelius Charms must have been added between the time that Harry arrived at Shell Cottage, and when he returned to it after burying Dobby.
But, this means that if you are within sight of the protected area before the charm is cast, you don't need to be let in on the secret, because we have now pinned down when the Fidelius Charm must have been placed on Shell Cottage, during the time Harry was off burying Dobby, HOWEVER, he did NOT need to be told the secret to return to the cottage. He just turned back to it and it was still there.
So, let's fully restate it one more time, with everything we know:
When a Secret-Keeper dies, their secret dies with them. Everybody in whom they confided will continue to know the hidden information, but nobody else. EXCEPT if you are within the premises of the protected area when the spell is cast, you do not need to be told the secret. AND if you know a secret place, you can bring someone who doesn't know the secret into the hidden place using side-along apparition. UNLESS the Secret-Keeper dies, then all those who know the secret become Secret-Keepers. UNTIL the spell caster dies, then the secret is revealed to everyone.
Knowing what we know now from the information in the books, this leads us to dark and ominous questions such as:
Would the Potters really had been any safer if Sirius Black had indeed been the Secret-Keeper? At some point would he have only had to tell Peter Pettigrew for Pettigrew to sell Sirius to Voldemort, in order for Sirius to die, and make Pettigrew a Secret-Keeper by default.
And, if Bathilda Bagshot had wandered into the garden of the Potter's home just as the Fidelius Charm was performed, would she too, like how Harry saw Shell Cottage, be able to continue to see the house and its residents without having been told the newly formed secret? Would she have kept the secret, even under the Imperius curse?
The flaws, the discrepancies, and in some ways, the reliance on mere chance, leads to the unnerving conclusion that if the Fidelius Charm really was the Potter's best chance of survival, then perhaps it was always only just a matter of time before Voldemort got to them, no matter who had been chosen as Secret-Keeper. If Sirius had lived, could perhaps this realization have lessened the guilt he felt for feeling responsible for James and Lily's death?
It would seem that the Fidelius Charm, on closer inspection, is not the powerful security measure that we were led to believe it was.
