The characters you as GM play can have the biggest effect on how the players experience your world and story. As the GM, you are the story-teller, and narrating the player's actions as they explore your world is the primary method of exposition the players receive; however, the players will be much more likely to remember a quirky character after a session than the way you described the mountains.

The best thing you can do when plotting out your story characters, is to assign them a core motivation. How does this character live? What do they need to survive or thrive? How do they intend to get it? What is getting in their way? How a character's motivations compare with the party's creates the story for you.

Make them Weird