For main characters I try to think of what I need them for, the purpose they serve, and then I try to make them have a particular/unique look to them so they don't look too similar to anyone else in the cast (unless they happen to be related). THAT's pretty conventional!

However, for background characters, I have a hard time designing them off the top of my head! So I have just resorted to opening the character customization feature in the game my comic is based in and hit randomize a bunch