Only two rules to learn about singing - if it's the same as the tune, it's unison. If it's different, it's harmony.. easy innit!

Oh, and a good scary thing if you can manage it, is to do the tune an octave up from the singer, particularly effective in a crowded bar, full of large men all singing butch and manly working songs. Come in, an octave up, preferably whilst standing behind one of the larger lads. The crowd will turn and look at him, and think that he has strained something. Oh, also helps if you have two volumes, loud and BLOODY LOUD. Being small and blonde means you can get away with this, although I have a 6 foot TS friend who has a 38c bust and sings bass......