Physiologically there is little to tell apart fear and excitement. They create similar outward effects – sweat, shallow breathing, adrenal response etc – but internally they feel totally different. one will invigorate you, the other will paralyze you. They are very close physically but diametrically opposed mentally. Learning to transform fear into excitement is one of the great keys to performance. Some people seem to do it effortlessly, for others it's a real struggle. It's not fatal if you can't, I've seen great athletes perform at the highest levels while afflicted with fear, but it is more difficult for them than for those who seem to relish the stage and see it only as inducing excitement without fear.
