Why didn't Lupin warn Harry not to use Expelliarmus in DH - The Seven Potters?

Question

In the chapter The Seven Potters in Deathly Hallows, Remus Lupin is appalled when Harry tells him Harry used Expelliarmus on Stan Shunpike because he felt Stan was under the influence of the Imperius Curse during the Battle of the Seven Potters. Lupin felt Harry should have used Avada Kedavra or Stupefy (if Harry wasn't prepared to take a life):

Lupin looked aghast.

‘Harry, the time for Disarming is past! These people are trying to capture and kill you! At least Stun if you aren’t prepared to kill!’

_{Deathly Hallows - page 64 - Bloomsbury - chapter four, The Seven Potters}

¯¯¯¯¯¯¯¯¯¯¯¯¯¯

‘So you think I should have killed Stan Shunpike?’ said Harry angrily.

‘Of course not,’ said Lupin, ‘but the Death Eaters – frankly, most people! – would have expected you to attack back! Expelliarmus is a useful spell, Harry, but the Death Eaters seem to think it is your signature move, and I urge you not to let it become so!’
_{Deathly Hallows - page 64 - Bloomsbury - chapter four, The Seven Potters}

If the Death Eaters believed Expelliarmus had become Harry's signature move, why didn't Lupin, or someone in the Order, warn Harry not to use that spell, should they encounter any Death Eaters?

Lupin was very vehement about it -- after the fact. Why not warn Harry beforehand?

Answer 1

Leaving aside the snide yet indubitably true "hindsight is 20/20", the answer is there in the quote:

‘but the Death Eaters – frankly, most people! – would have expected you to attack back! Expelliarmus is a useful spell, Harry, but the Death Eaters seem to think it is your signature move, and I urge you not to let it become so!’

1. Lupin is quite likely one of those "most people".

   The very fact of his anger and lecturing at Harry ("Lupin looked aghast") is evidence that Expelliarmus was NOT what he himself expected of Harry (note that he's pretty adamant; instead of merely "what else could I have expected" resigned).

   So, he (nor anyone else) did not make the warning because they didn't know it would even be needed.

2. This is NOT his "signature move" yet before they take flight; Lupin explicitly talks about establishing a pattern in the future.

   Please note that the whole "signature move" is a pretty thin pattern

   • mostly based on ONLY TWO before-Seven-Potters fights (GoF duel with Voldemort; and one case of casting it at the Ministry in OotP. Literally, just one).

   • In other previous duels that Death Eaters would be aware of, he used other spells (Sectusempura against Malfoy, Cruciatus against Bellatrix, Protego, Reducto, Colloportus, Stupefy, and 3 or 4 Petrificus Totalus in the Ministry).

     To re-iterate, "big data" analysis would point to Petrificus Totalus being Harry's Signature Spell, not Expelliarmus

   • Lupin knows more than most people about it because he taught Harry the Dark Arts, knows Harry well, and probably heard of his Expelliarmus lecture at the first DA meeting.

   Voldemort is brilliant enough to figure it out... but he's Voldemort.

3. Just to be clear, the "If the Death Eaters believed Expelliarmus had become Harry's signature move" is not only hypothetical (note the "if") but, as discussed above, is and opinion mainly based on what happened in the battle of Seven Potters.

   Meaning neither Lupin nor anyone else would have thought this is the case until Harry was "outed" based on Expelliarmus by Voldemort.

Answer 2

Keep in mind that Lupin is speaking with the gift of hindsight. Things that seem obvious in retrospect aren't always obvious at the time. ("Of course invading Russia was a bad idea! Sure, if it had worked Hitler would have easily won the war, and Russia was going to turn on them anyways. But come on!)

The Order expected only token resistance when they took Harry from his aunt and uncle's house, and certainly did not expect that Voldemort would be present:

"We’ve leaked a fake trail to the Ministry: They think you’re not leaving until the thirtieth. However, this is You-Know-Who we’re dealing with, so we can’t rely on him getting the date wrong; he’s bound to have a couple of Death Eaters patrolling the skies in this general area, just in case."

Under the Order's original plan, Harry using Expelliarmus would have been a moot point. One of the few Death Eaters would have to be lucky enough to chase the real Harry (who they intentionally placed with the weakest protector). And at that point, Disarming would have effectively shut down the only threat to Harry's safety.

But really, I get the sense that Harry revealing himself using Expelliarmus was the smallest part of Lupin's concern. He was more concerned that it showed that Harry wasn't taking the risk to his life seriously, and he was too "soft" on enemies that wanted to kill him. But Harry's worldview and refusal to kill his enemies is largely vindicated when his "signature" spell is the one he casts to defeat Voldemort.

Answer 3

I would like add another thing to DVK's answer, especially Point 2 (Not yet a signature move).

A signature move is something which allows the executor to be identified. Why does the executor use a signature move at all because identification in it is a disadvantage ?

Because to grow the move as habit he/she must be very trained in it. That means he/she is very experienced in the move und will it perform with outstanding perfection in contrast to other available moves. This means it is very likely the thing which simply works excellent for him/her and its usage increase the knowledge about the countermeasures, giving further incentive to concentrate on the move. All this means that if under duress the move will likely the thing which will be executed automatically even in situations where it does not fit. It also means once you have such a move people can prepare for it.

Lupin is aghast not because the Death Eaters thought that Expelliarmus is Harrys signature move, but because they were really able to correctly identify him with it. This means that they may have a point and Harry is really in danger to use it as such. And he is angry because Expelliarmus is in his opinion a very stupid move to use; that Harry used it at all in the situation under duress is another indicator that the Expelliarmus could become Harrys signature move.

And to Lupin's defense, Harry actually uses it in the final duel.