I've actually learned to recognize when people expect me to finish a sentence for them and then just wait for them to finish it themselves.

Them: "Hi, I'm looking for a Stephen King book, and I was wondering if you could..."

Me: "..."

Them: "..." [meaningful look]

Me: "..." [blank questioning stare]

Them: "...help me find it?"

Me: "Oh, of course! It's right this way."

It's a fun little game to play with conversational expectations.