Of course you don't.. just be convincing. Otherwise we are suggesting that anyone who plays Hitler or Charles Manson in a film has to believe in the things they did which is clearly rubbish. You may well enjoy singing a great song that extols the joys of cocaine ... or hunting without agreeing with its sentiments. Over the hills and far away is a great song ... but its a recruiting song for the army.. you can be a pacifist and still enjoy singing the song.... this wouldn't make you a "hypocrite and a liar" as was unkindly suggested above.