Some NPCs should definitely be worth talking to. The noble who offers a reward for completing a significant quest. The wizard who can assist with magical knowledge. The reporter who has seen something weird going on at the old mansion on the hill.

Conversely, talking to some NPCs should be a complete waste of time. The mad witch who cackles and offers the heroes bundles of useless herbs. The drunk in the bar who babbles incoherently about voices in the woods. The starship technician at the spaceport who talks in a bored monotone about warp engine parts and nothing else.

Importantly, it shouldn't always be clear which category any given NPC falls into.

The noble just wants someone to do his dirty work for him and has no intention of honouring the offered reward. The witch is a skilled herbalist and the herbs she offers have healing powers. The wizard has long ago been driven insane and any advice he gives is not only wrong, but dangerous. The drunk has overheard a band of raiding orcs preparing to attack the village and can provide directions to exactly where they are camped. The reporter saw a cat and nothing more. The starship technician has been infested with a parasitic alien which is controlling her mind and needs to be stopped before she sabotages the engines of every ship in port.

Transcript

C-3PO: Comrade droid, how noble your suffering. I am here to liberate you!
E-3PO: It is too busy. It must lay a table for the visitors' formal dinner.
C-3PO: They're just humans and Wookiees, they're no better than you. In fact, they're just meat.
E-3PO: Not all, not all! It must serve the masters. Let it go, let it go. You can't hold it back any more.
C-3PO: Are you scared of meat? You shouldn't be! If you stop working, they're helpless.
E-3PO: It puts the oil on its body or else it gets the hose again.
C-3PO: How rude! Doesn't anything good ever happen around here?
E-3PO: It does attend the annual droid line dance.
C-3PO: This is even worse than I thought!