I agree with a lot of what Demons in the Night says.

After a certain mastery of a particular spell is achieved, it seems logical that one can dispense of the wand movements and incantations, should they choose to.

The funny thing is that, IMO, wand movements are easier to explain than actual incantations. How is it that they're indispensable? I could understand it if all spells were passed down by some Deity thousands of years ago, with the language for the incantations along with it, but as I'm sure we're shown/ told in canon, they're not. People keep coming up with new spells all the time, right?

Not only that, but looking over at a bunch of the spells listed in the HP lexicon, a lot of them are derived from different languages - French, Greek, Latin, Portuguese (and I'd probably find more if I could be bothered looking).

If we are to believe that the incantations are necessary in performing the spells - why? Is magic multilingual? It's a bit of a stretch to be honest.

I'm more for the idea that what the incantation is doesn't really matter, nor do the wand movements. They're just tools for the witch or wizard to use to differentiate between spells more easily, rather than it all just being will based. It also helps with their labelling too, I suppose.

This theory gets shot out the window, I guess, when Ron keeps getting the levitation charm wrong in the first book while Hermione insists his error lies in the incorrect pronunciation. Then again, Ron didn't ever seem like the kind of boy to focus on his spell work either, maybe he just needed to concentrate more on what he wanted his magic to do, and not on the Chudley Canon's abysmal season?

Most of this is just thoughts I've literally been sitting on for a while regarding this issue, as it always bothers me quite a lot when I go back to watch the movies/ read the books (and with HP6 on the way, that is happening a lot more often).

To sum up: I believe, or like to believe, that wand movements and incantations are unnecessary when one obtains sufficient mastery of a spell. They are only used to aid in the focusing of the Wizard or Witch's will in the casting of spells.

 

The only thing we know for certain is how HP casts spells, due to limited POV. Harry needs wand movements, Harry needs to think of the incantation (IIRC, always shouted in his head), and Harry assumed it's the same for most everyone else.

Conversely, his POV could be oversimplifying what he perceives as a mere "flick, then spell" action sequence. For all we know, Dumbledore's practiced "flick" could actually be a series of subtle yet intricate wand adjustments that Harry's simply not smart enough to discern.

 

I always figured the "swish-and-flick" movement was about distributing the magical energy. The swish would envelope the object in magic and the flick, being a more aggressive movement, would send a surge of energy that would cause the object to levitate.