There is another point - on choruses.
It is also good manners to listen to what the singer is doing and not just 'do your own thing', especially in regard to speed.
We saw Walter Pardon perform at clubs on many occasions and we know he quite liked people joining in on his choruses - but....
He actually abandoned singing two of his songs because of audiences' tendencies to low down and drag out the choruses, leaving him to have to pick up the speed he wished to sing at the beginning of each new verse - the singer should be allowed to set the pace - it's his or her song.
I gave up singing 'Go to Sea No More' in public when audiences insisted in putting in a chorus, which, for me, made the song (at the time, my favorite)interminable.
The Singers Club was great for choruses, especially when Ewan and Peggy were on because they made a point of teaching choruses and sometimes experimenting with them.   
Ewan's ballad, 'The Baron of Lys' has a section with a question-response chorus between the seduced woman and the seducer - Ewan divided the audience up into genders and got the women to sing the first part and the men, the second - great fun when it worked.
Peggy regularly suggested that some of here choruses would take harmony "if you feel up to it"   
Jim Carroll