I may have an unpopular opinion, but I like it when people finish my sentences. It may be a little embarrassing and humiliating, but as soon as they say the word, I am immediately fluent and I get the word out. I like to pretend I can’t remember the word, and so I don’t draw too much attention to my stutter. But it’s purely subjective. For you, just wait it out. It tells the stutterer that you’re very patient and it’ll help ease their nerves.