There are several ways to create the feeling of nonlinearity in an RPG, and these are through character interaction, level design, and consequences for players' actions, primarily. Nonlinearity is not the same as completely open-ended gameplay, but rather, it's the feeling that your actions and choices change the course of the game (though you may still need to perform specific quests in a certain order). On the flip side, in an open-ended game, you can do anything at anytime.