Most of what has been said applies to a 'performance to an audience' in a pub. Once uponn a time, sing songs in pubs, where there is no demarcation between performer and audience, were popular, I used to go to a great friday night sing song in Anstey, Leicestershire, about 25 years ago. I still do them occasionally, its a matter of getting the feeling of the crowd, starting with stuff that people are likely to know, doing stuff with easy join in lines, and inviting all and sundry to start a song. No mikes. Even the slightest bit of PA is an invitation to talk rather than listen or join in, and move around the bar as well.