Bait and Switch

Players have a tendency of refusing a call to adventure or plot hook that is too big or doesn't fill their coin purse. So what is a Game Master to do? Well, there is always the refusal of the call and the consequences there. This is viable and certainly can be a source of good gaming in itself. But there is another method, one used by authors (the original Game Masters) since time immemorial.

The Bait and Switch
Frodo Baggins did not sign on initially to sacrifice himself nobly for Middle Earth. He agreed to transport a magical ring to Rivendell for Gandalf the Grey. Luke Skywalker did not sign on to depose the Emperor and help kill his father, he simply helped two droids find a Master Obi-Won Kenobi, and then agreed to help Obi-Won rescue a princess. Jake Sully signed on a mediate a peace agreement by piloting an Avatar body, not fight a revolution. At ever stage the people hiring the hero's lie or stretch the truth so that the hero will help them. Game Masters can do this too.

Killing NPCs
Keep in mind though, that players have a lot fewer inhibitions than heroes in most other stories, and so a Game Master needs to make certain that they have either protected the lying NPC against player revenge, or have made certain that the plot will progress without said NPC. Because after all, players will surprise you