You need to plan your performance to match your audience, even if it's only one song. Especially if it's only one song, so the audience gets that you're awesome. There are different dynamics of performances, usually moving between the opposing poles of deep personal expression (I'm just here singin' my songs, telling it like it is, opening my heart and O, look there's an audience here, I guess you guys can stay - think Sarah McGlaughlin, John Mayer, Debussy) versus We are rockin' the house TONIGHT, step back and get down! - think Garth Brooks, Lady Gaga, Beethoven). Think of the age and interests of the people you're playing for. Honestly, this is pretty important just playing for friends or family. Your grandmother isn't going to be able to relate too well to your Korn songs, but she'll think you're cool if you play a song closer to where she's coming from. Think about how your performance meshes with your audience.
You can only take your audience one or two clicks further than where they are right now without losing them.