I was on a panel, and you never get to say all you need to, so I am going to do a series of posts about them:
#1 Your main character should be the main character with the most to lose. Interesting characters can be interesting because they have nothing left to lose. We are fear based creatures that operate on a loss basis, not a potential gain
#2 your most unforgettable characters are going to be the one with the most alien POV. Great for one liners, loses its punch quickly if it’s the voice.
#3 murderability. That is so a word you almost can never kill off the mc without problems in the plot. You can threaten to kill the interesting character and have your reader willing to believe you’d go through with it. Please don’t kill off my favourite character though. Please.