A discussion of fanfiction.
Fanfiction is great for beginning writers. The characters and settings are already established, so all the person has to do is come up with a plot. It's a good learning tool to get a new writer into the flow of writing.
But fanfiction is like training wheels. You have to learn to let go of it if you want to become a serious writer. You can never publish fanfiction (unless you're the author of 50 Shades, but don't even get me started on that abomination) because of copyrights.
Fanfiction is the easy way out for a writer. They don't have to be original or come up with their own ideas. This is like poison to a serious writer. They get the characterization and setting handed to them on a silver platter, and they may end up forgetting how to do it on their own.
If you want to become a serious writer and hope to be published someday, step away from the fanfiction and start writing original fiction. Come up with your own plot, characters, and setting and be taken seriously as a writer. (Don't get me wrong. Serious writers can most definitely write fanfiction if they want to, but it should be purely for fun. Don't think you'll ever get anywhere in the literary world by just hashing out fanfiction, no matter how original you may think it is.)
The fine line between inspiration and plagiarism.
Copying someone's plot or characters and mixing them up is not original fiction. It's plagiarism. Finding something you like, trying to imitate it, and calling it your own work is plagiarism.
But it's not always that easy to discern if you're blatantly copying something, or it inspired you. Look at the movie, Avatar. The entire plot was exactly the same as Pocahontas (and there are a ton more stories with that exact same plot). Was it plagiarism? I'd say yes, but countless others would argue against that.
We all find inspiration from books and movies but at what point does it turn from inspiration to stealing ideas? It's impossible to find exactly where you cross that line, but I'll offer tips on how to be as far away from that line as possible.
How to come up with a unique idea.
Here's my advice on how to not copy something. For example my novel, SuperHero, pulled inspiration from the movie V for Vendetta. Inspiration should be taking the feel of the inspiration rather than the exact element itself. In my case, I loved the idea for V's mask being a symbol for the people rather than just a way to hide his identity, so I gave my superhero a mask even though everyone already knew who he was. I also designed the mask so it looked completely different from the Guy Fawkes mask in V for Vendetta. Still, I had a short monologue spoken by the hero about the mask being a symbol, but I didn't copy V's monologue. I listened to V's speech, and a few days later, I wrote that scene in my novel. That gave room for my brain to insert its own ideas into V's speech and rephrase the words in a way that fit my character.
If you watch a movie and find something they did or said was awesome and want to use that idea in your story, take a few days off from writing before you start on that scene. That will help you morph the idea into your own. The heart and organs lie at the core, but the outfits and skin are different.
Don't try to copy and ENTIRE storyline. Take bits and pieces from a ton of sources. If one movie inspired your story, great! Now find similar movies or tv shows or books and mix them together. A story is just a mixture of ideas pulled from a ton of places. We see those Twilight-rip-offs all over the YA section these days, but why do we call them rip-offs? Because it seems like Twilight was the ONLY inspiration. They didn't have any other material to pull ideas from, so it turned into a carbon-copy story.
So back to my novel as an example, I pulled inspiration from the Holocaust, V for Vendetta, the song Hero by Skillet, the video game Deus Ex: Human Revolution, Dragonball Z, Iron Man--and those are just the big ones. The wider the variety of places you pull inspiration from, the more unique your idea will be. It's not WHAT you take from another source, but HOW you put all that information together that makes your story unique. Each story is just one rehash of the 26 letters of the alphabet strung together in different combinations. Don't let your story have the same combination as another.