Maybe you could play songs that are harder to sing, like “Take On Me” with that falsetto screech in the chorus.
Follow it up by “Blinded by the Light,” where you know they’ll botch the lyrics nobody can ever make out. “Wrapped up like a dude another rotor on the right…”
Then work in a song completely inappropriate for the singer, so that a burly 300-pound fellow has to belt out “I Feel Pretty,” assuming you’re not in a venue where that would be a normal thing, in which case you pull our your heavy metal fakebook.
Finally, you can play lots of songs that are duets, and hope they don’t have a buddy ready to join in.
This is interesting. I have had this happen many times in the past because I played songs that people knew. My instinct was to slow down and follow them, that is the accompanist in me I guess.
A weird one that happened was I was playing before a wedding. I have an arrangement that I did of Chi il bel sogno di Doretta from Pucinni’s La Rondine. Not many people know this beautiful aria, but from behind me, as I was playing, this woman began to sing, what a lovely voice she had! I continued to play and she sang never missing a beat!
Just be glad you play the pedal harp, David. What I really, really hate is when I’m playing, say, “Moonlight in Vermont” or something and someone starts singing behind me and I know it’s Vic Damone or Rod Stewart or Tony Bennett or someone like that, because I know what’s coming–everything is just absolutely swell till the bridge and then I hear, ‘Hey! It should modulate to [key of their choice] there!”