A game is just a system of rules, but a role-playing game is more than just rules. Role-playing is acting as if you weren't yourself. Though speaking in-character is a hallmark of role-playing, you don't have to be an actor. Staying in-character means only that the other players are understanding your actions through the lens of the character you're playing.

Even Monopoly can be a role-playing game if you make decisions based on an imaginary person's personality rather than simply what you believe is the best course of action to win the game. You could start speaking in an accent you imagine a wealthy tycoon would have, or you could start renaming the places you buy the way a real tycoon would. A role-playing game's system, then, is merely a way of creating just enough of a hint for you to create a senerio of your own to act out or describe.