So, the characters created by the real author.
Things to do:
1) Make them act realistic, and like they do in the original material.
Say, Percy Jackson: Leo needs to be flirtatious, annoying, and a sarcastic little sh*t. Percy needs to be loyal, and halfway stupid. Annabeth needs to be BLONDE. End of story.
Hunger Games: Katniss needs to be stubborn, and serious. Peeta needs to be sweet, and caring. Gale needs to be a jealous mofo.
Twilight: ((just examples)) Make Bella this wimpy b*tch who can't make up her mind. Edward is this sparkly guy, who may or may not actually be gay.
Mortal Instruments: Jace is sexy and sarcastic. Clary is short and artistic. Simon is another sarcastic mofo.
Get my drift?
2) People tend to like fanfiction that is like an extension of the books, so, give the PJO crew a new quest, continue the story after Mockingjay.
3) Make your original characters mesh into the story.
Don't be writing a story about Percy Jackson, and throw some witches and One Direction in there.
Just...no. One Direction is the root of all evil. My best friend, @sharpshooter78, and I decided that we will murder everyone who writes 1D fanfics.
