During a live gig, the relationship between the artist and audience is beyond that of simply performer and observer. Physically and emotionally, there's little separation between both parties and a huge deal of direct interaction, both explicit and subliminal. Self-assurance is essential and, as with so much in life, confidence is the key. If a band looks like they're enjoying their music and project a sense of confidence, that will convince a crowd as much as their material will. Conversely, bands that look shy and embarrassed will make the crowd feel awkward, and that negative emotion becomes associated with their music.