Why doesn't Lupin summon a patronus on the train?

Question

In the third film (Harry Potter and the Prisoner of Azkaban), when the Dementor enters the train carriage, Prof. Lupin casts the silent spell which is obviously Expecto Patronum. However, he only manages to conjure the whispy shield, rather than a fully formed (corporeal) patronus. Even though it is shown later in the film that he's actually extremely skilled with the spell.

Did he only use a weak memory to not get a patronus running around the carriage? or was it just because he had woke up and didn't think fast enough?

Answer 1

I don't know about the movie but this is the passage from the book (Hermione telling Harry what had happened):

　"And Professor Lupin stepped over you, and walked toward the dementor, and pulled out his wand," said Hermione, "and he said, 'None of us is hiding Sirius Black under our cloaks. Go.' But the dementor didn't move, so Lupin muttered something, and a silvery thing shot out of his wand at it, and it turned around and sort of glided away.... "

• It doesn't specifically say that it was a full bodied patronum but it doesn't really say or indicate a wispy shield.
• Even if it wasn't full bodied we could try to make sense from the context. If he doesn't want to put too much effort into it, like shooing away a cat; First you say shoo! then you say Shoo and wave your hands and only as a last resort would you get up and chase it away ;)

Answer 2

From JK Rowling's entry on Patronus on Pottermore

In some cases a witch or wizard may choose to produce an incorporeal Patronus deliberately, if he or she wishes to disguise the form it generally takes (Remus Lupin, for instance, is afraid that his corporeal Patronus gives too much away).

Answer 3

The Pottemore entry on Remus Lupin explicitly answers this:

Remus’s Patronus is never revealed in the Potter books, even though it is he who teaches Harry the difficult and unusual art of producing one. It is, in fact, a wolf – an ordinary wolf, not a werewolf. Wolves are family-orientated and non-aggressive, but Remus dislikes the form of his Patronus, which is a constant reminder of his affliction. Everything wolfish disgusts him, and he often produces a non-corporeal Patronus deliberately, especially when others are watching.

Answer 4

The Patronus doesn't have to take the animal form to be effective. But it does seem to still qualify as a Patronus even if it doesn't resemble an animal.

During Gryffindor's Quidditch match against Ravenclaw in Prisoner of Azkaban when Harry sees the demontors (that weren't demontors), he performs the Expecto patronum spell. What "erupted from the end of his wand" is:

Something silver-white, something enormous

No mention of an animal form. Despite that, Lupin tells Harry:

That was quite some Patronus

Again no mention of the animal.

Quotes above from Chapter 13, Prisoner of Azkaban (Scholastic paperback) pages 262 and 263.

Answer 5

Remus wanted to keep it secret from the students that he was a werewolf. There was a good reason for this, as he explains in chapter 22 of Prisoner, after his illness became public.

‘This time tomorrow, the owls will start arriving from parents – they will not want a werewolf teaching their children, Harry. […]’

Had Remus produced a corporal patronus, it would take the form of a wolf, which could be too much of a hint for students. He could drive the Dementor away with a non-corporal patronus, so he deliberately chose to produce that and not risk a reveal.

Answer 6

I don't have any real evidence but my presumption is that it was only 1 Dementor so he didn't need to use a full patronus. The only reason for that logic is when Harry was

trying to protect Sirius he was able to use a "whispy shield" of a patronus to deflect a couple of Dementors.

Answer 7

Well, as Izkata said, he was asleep just a moment ago, and I doubt anyone could conjure a strong enough spell right after waking up, although he is a skilled wizard.

A Patronus can be strong as a Corporeal Patronus or a Non-Corporeal Patronus. In the movie, nobody except Harry could cast a Corporeal Patronus, and in all the movies, Lupin has never shown that he can cast a Corporeal Patronus.

Answer 8

In regards to Professor Lupin's Patronus on the train. I wondered too why Lupin's Patronus didn't seem to take full form, after thinking about it, the answer may be very simple. The book does imply something came out but wasn't very specific, that being said, it may be as simple as this; Lupin was sleeping on the train when the dementors started their attack. Lupin's Patronus may have been somewhat stymied because he wasn't fully awake. We all know when we wake up all of a sudden, from a good sleep because of some sort of noise or commotion going on we are somewhat disoriented, this could explain the somewhat partial formed looking Patronus.