This is not so straight forward, but seems to fit the bill:

Sid and Nancy in Albert Herring undermine Lady Billow's grand village bunfight by doctoring Albert's orange juice (or am I muddling the drink up with Gussie in Right Ho! Jeeves?)  They are both shop assistants.

It's lemonade (accompanied by the Tristan love potion motif )

Significant characters who appear on stage but do not sing?

Significant characters who appear on stage but don't sing:

The Tsar in WAR & PEACE
The Spanish Ambassador in EVGENY ONEGIN
Sorrow in MADAM BUTTERFLY
Lillas Pastia in CARMEN
Toby in THE MEDIUM
Clown, Stagehand, Policemen, Lottery Agent, all in LULU

Significant characters who have spoken roles:

Prince Menshikoff in Gretry's PIERRE LE GRAND
Colonel Cohenburg, the hero of Storace's THE SIEGE OF BELGRADE
The Theatre Director in Mozart's SCHAUSPIELDIREKTOR
Alexei the Fisherman, and all five visitors in Verstovsky's ASKOLD'S GRAVE (1835, Bolshoi Theatre - forgotten major work!).
The Pasha in Mozart's ZAIDE (it's a play with music, so this is hardly surprising)
Vampire Master, and Lord Ruthven's servant John Pearce in Marschner's DER VAMPYR
Orpheus in Dargomyzhsky's ORPHEUS (the entire role is recited over the background of the music)
Police Commissioner, LULU
Narrator, OEDIPUS REX

Time for a new question, I think??

Tell us about operas whose plot hinges on a special effect*?

* ie lighting or scenery..  not costume-changes, we've done that recently

The most obvious goldmine of special effects would seem to be the Ring, what with rainbow bridges, magic fire, winged horses, giants (who sometimes look like dragons), dwarves (who sometimes look like monsters or toads), underwater-dwellers, and the small matter of immolating and then quenching the whole world as we know it... (having re-read the above list, I see that the presence of a Tarnhelm alone really has a lot to answer for!)

Could I include something as basic as the lighting in Act 1 of La boheme?  If it wasn't dark, Rodolfo couldn't nick Mimi's key and use it as an excuse to keep her talking... and then the moonlight comes through the window and makes her look all gorgeous and ethereal and induces him to sing O soave fanciulla...

(Though it's really shocking how unsympathetically some productions of the above are lit.)

What counts as special effects?  Minotaurs' alter-egos appearing to them in dreams?  Statues coming to life and dragging morally bankrupt noblemen into the fires of eternal damnation?  (There are other fires in opera, too - The Pearl Fishers ends with Zurga setting everything on fire so Nadir can escape with Leila...)

There's the famous collapsing bridge in La sonnambula (and if it's the ENO production, one in Serse too )

The collapse of Klingsor's kingdom in Parsifal

And anything involving transformations: Daphne, for example.

And has anybody ever seen La Wally staged with a genuine avalanche?

The Ring, certainly.  I personally long for the day when I can see Fricka's chariot hauled by real rams ...  and of course there are tenors walking through walls of impenetrable fire, woodbirds, dragons (I once years ago managed to see WNO's Siegfried and Zauberflote within a few days of one another, and the beast that chased Tamino on stage was quite obviously Fafner from a week earlier)

And then there are all the shipping movements in the first scene of Der fliegende Hollander; and swans pulling boats and turning into knights in Lohengrin  (the sequence of events at the end of Lohengrin is that the swan sinks into the Scheldt and Gottfried then emerges from the river just in time to catch the body of his expiring sister, while a passing dove gets - literally - roped in to hauling Lohengrin back to Montsalvat.  You begin to appreciate the attraction of abstract production)

Special effects are often the downfall of Parsifal - both the dead swan thumping down on to the stage in Act 1 and Klingsor's throwing of the spear at Parsifal, who catches it, are moments that can raise more than the odd snigger in the audience.

Another WNO memory - what looked like a large rubber chicken being dropped from the flies, landing at Gurnemanz's feet with a resounding thump.  A few inches to the right and it would have landed on his head.

Another opera that involves a range of potentially hazardous special effects is Die Frau Ohne Schatten - involving singing fish in a frying pan, emperors turning into stone and back again, waterfalls, labyrinths, storms and the need to portray Barak's brothers - the One-Eyed, the One-Armed and the Hunchback - in a way that doesn't invite ridicule.  Beside that lot, the singing conch-shell in Die Aegyptische Helena is a doddle ...