Sooo many times I have read fanfictions where the author has characters act differently from how their personalities would dictate. It's very annoying. Characters should stay in character. Writers shouldn't change the character just to fit their own needs/wants for the story.

Though imagination is important too.

And to do that you have to think like that charicter, you have to think and act like that charicter and feel there emotions (Many times I had to think like Scar, (The villian from the lion king)) And somtimes it's hard, so I play different soudtracks from the movie that I'm basing my fan fiction on, like if I'm writting a fanfic about Scar I would listen to his song, (Be Prepared in the backround) And when I came to a diffrent charicter I would listen to a more hero like or heroin like song.

When I write fanfiction, I watch the characters' movies and scenes over and over to keep the characters' personalities fresh in my mind.