Rule: Spend Inspiration to earn Advantage AFTER you have rolled the first die.
D&D 5e has an “Inspiration” rule that lets you earn a quality called Inspiration through role-playing, specifically by playing up your character’s Bonds, Ideals, Flaws, or Traits (BIFTs). You can only earn one point of Inspiration at a time, and you must spend it before you can earn it again. You may spend your Inspiration to earn Advantage on a roll, which means you roll 2d20 and take the higher result.
As written, the rules require that you state that you’re spending your Inspiration before you make the die roll. In my games, you can spend your Inspiration to earn Advantage after you’ve rolled the first d20.
My reasoning is that I want to encourage players to role-play more. If you have to spend Inspiration before the roll, you’ll hold back and do it less often, so you won’t be able to earn Inspiration again. By letting you spend Inspiration after a failed die roll, you’ll use it more often, which means you’ll need to earn it back more often, which means more role-playing.
Even if you were going to role-play a lot anyway, the Inspiration mechanic gives recognition for role-playing (you can earn it from the DM or from another player). Recognition is even more important than getting the extra die.