What really happened on the tower.

[justcarol67]

No: HPFGUIDX 155651

Carol earlier:
<snip> And we *are* told that Harry didn't have time to raise his wand
before Fenrir Greyback leaped on him, which (along with Greyback's
rancid breath in his face) strongly suggests that Harry was in no
position to cast the spell himself.
> 
Ken responded:
> I don't recall how I interpreted this passage when I first read it.
My second and third readings were influenced by those who claim it
wasn't Harry who cast the PT. Reading it with an open mind again as
the result of this discussion leads me to believe there is no reason
why Harry could not have cast the spell. Yes, Fenrir was on him and
knocked him down *before* he could raise his wand. It does not say
that Harry's arms were pinned helplessly to the ground. It does not
say that Harry could not raise his wand *after* being knocked over. It
does not say that Harry could not move his wrist enough to point his
wand at some part of Fenrir, which is all he would have to do. It
might have been someone else who cast the spell. The simplest reading
that fits the text is that Harry cast the spell. It could not have
been Snape, he and Draco had already disappeared around the corner.
> <snip>

Carol responds:
Let's just say that the unreliable narrator has set us up once again:
We don't know who cast that spell, and it looks to me as if Harry is
totally helpless and facing death as Fenrir's ugly face comes close to
his. We agree that Harry could not have cast a nonverbal spell. I
suppose it's possible that he cast a verbal one, but since the
narrator says that he felt Greyback's body roll off him, I don't think
he's the one who cast it. (If he cast it himself, why wouldn't the
narrator say so?)

Also, although Snape has rounded the corner, he's also making sure
that the DEs are following him, and he could, unknown to Harry, have
looked around the corner, shot off that spell, and continued on his
way. Tonks says that she heard snape shout something: Harry says that
he shouted "It's over." Tonks appears to accept that explanation, but
that doesn't mean Snape didn't also shout "Petrificus Totalus!" JKR is
very good at providing partial explanations or misdirecting the reader
with near-simultaneous actions. Case in point: Harry and Hermione hear
a noise behind the shelves. Madam Pince comes out and yells at Harry
for "desecrating" the HBP's Potions book. Later we find out that Draco
was also behind the shelves, listening to Hermione telling Harry that
Filch couldn't detect a love potion discussed as perfume or a cough
potion (a bit of knowledge that he uses with regard to the poisoned mead).

So I wouldn't rule out Snape as the caster. It could also be an Order
member, of course, but why would having his life saved by, say,
McGonagall be any big deal?

Carol, agreeing with the part of your post that I snipped about Harry
needing to learn to cast nonverbal spells