I've never heard the word 'actress' used in a way that implies less seriousness. I've never heard 'heroine' used in any way that wasn't heroic. The female lead maybe, but no more than 'hero' is used for a male lead and never as the person getting rescued.

I think the feminine versions of those words are used when someone is trying to be clear that they're talking about a woman. The origins of the terms probably come from English being a language of mixed origins. Some of our words come from languages that had male and female versions of things, some not. So there's no consistency.