I usually find some mechanic I want to experiment with or explore then build everything around it. Like finding a feat that sounds cool, or a subclass ability that would be fun to try. I find it difficult to pay attention if I'm playing something simple where I just walk up and hit things with the same attack time and time again, no matter how much I like roleplaying the character. But if I find a cool ability I'll always be looking for new ways to use it and be a more active player in the game.

Plus it gives me a good place to start fleshing out the flavour by justifying why they would have that ability and what it would look like.