All a listener has to go on is their own perceptions. If a performer is intentionally singing a certain way to get the emotion across doesn't really matter. It's whether or not it SEEMS that way.

Someone can mean every word they sing and just be putting their own feelings into it and come off as a manipulative ham. Another person has carefully thought out the song - "at this point I will get very quiet and slow down to show sadness" - but does the song with finesse. You're so into the song that you don't notice. I guess the really unforgivable sin is being unsubtle.