The whole "sing-along" thing is very much subject to cultural variation in my experience. Here in Maine, people are generally less demonstrative and getting an audience to sing is difficult. Upon reflection, this may be because the singing tradition here is largely solo ballad singing with few choruses. People would gather to hear the story being told in the song and not interrupt the singer, similar to Travellers we have heard in Scotland.
A few years ago, we started our tour in Machias, which is way "Downeast" in a relatively conservative area. We felt we had moved the audience when they actually unfolded their arms and chuckled occasionally during a humorous bit. A few days later we did pretty much the same set in Maryland and people went wild, singing on choruses, stamping their feet during peppy songs and laughing uproariously at the same humorous bits.