In the Harry Potter books, magic cast by a wand is often accompanied by a visual or audio special effect, described in varying levels of detail as either a jet of light, a jet of sparks, a puff of smoke, a flash of light, a popping/cracking/banging sound, or no visible or acoustic effect at all.
These effects are often inconsistent in appearance and sound, as in the case of the Disarming Charm, which is described as appearing as either a jet of red light, a banging sound or as completely silent and invisible on different occasions.
Snape cried: "Expelliarmus!" There was a dazzling flash of scarlet light and Lockhart was blasted off his feet ... - Harry Potter and the Chamber of Secrets, Chapter 11, The Duelling Club, pg 142, Bloomsbury Edition.
Harry reached his wand just in time. Lockhart had barely raised his, when Harry bellowed, "Expelliarmus!" Lockhart was blasted backwards [no mention of spell effect] ... - Harry Potter and the Chamber of Secrets, Chapter 16, The Chamber of Secrets, pg 220-221, Bloomsbury Edition.
Before Snape could take even one step towards him [Harry], he raised his wand. "Expelliarmus!" he yelled - except that his wasn't the only voice that had shouted. There was a blast that made the door rattle on its hinges [indicative of sound]; Snape was lifted off his feet and slammed into the wall ... - Harry Potter and the Prisoner of Azkaban, Chapter 19, The Servant of Lord Voldemort, pg 265. Bloomsbury Edition.
Voldemort was ready. As Harry shouted "Expelliarmus!", Voldemort cried, "Avada Kedavra!" A jet of green light issued from Voldemort's wand just as a jet of red light blasted from Harry's ... - Harry Potter and the Goblet of Fire, Chapter 34, Priori Incantatem, pg 575, Bloomsbury Edition.
Harry moved off into the middle of the room. Something very odd was happening to Zacharias Smith. Every time he opened his mouth to disarm Anthony Goldstein, his own wand would fly out of his hand, yet Anthony did not seem to be making a sound ... Fred and George were several feet from Smith and taking it in turns to point their wands at his back. "Sorry, Harry," said George hastily, when Harry caught his eye. "Couldn't resist." - Harry Potter and the Order of the Phoenix, Chapter 18, Dumbledore's Army, pg 349, Bloomsbury Edition. [This suggests that Expelliarmus is invisible and soundless, for Smith not to notice the direction from which the spell was coming].
I figure that this is simply the nature of magic - unpredictable and illogical at times. But is this right? Does Rowling keep these things a little vague and ambiguous just so that we, the readers, can decide what we want the spells to look and sound like ourselves? Or has she explained the rules of spell effects in an interview or through some other medium (e.g her website)? If so, can someone please tell me what the rules of spell special effects are? When does a spell appear as a flash of light and when a puff of smoke?
P.S. I sometimes wonder if spells with no apparent special effect (e.g. Cruciatus Curse, Imperius Curse) actually do have an effect imagined by Rowling. If so, then I would guess that Rowling leaves the descriptions blank so that the readers' imaginations can fill them in. Has Rowling said anything to support this idea?