Original Characters in Fan Fiction

Outlook and Personality

Although a great deal of personality is innate, a person's basic character is also moulded by experience. And a lot of those experiences come from culture. That isn't to say that your characters always must follow cultural expectations - but if they break away, they'd better have a good reason.

For example, imagine you're writing a Star Trek story, and you have a Vulcan OC. As Mr. Spock would tell you, Vulcans prize logic and self-control (1). So you probably shouldn't write a scene where your Vulcan OC calls the ship's captain a 'terran buffoon', then bursts into tears over a hot fudge sundae on the holodeck. Unless, of course, this outburst is the first symptom of Bendii syndrome (2), and your story is a commentary on how we deal with dementia.

Make sure your character knows only things that are actually possible to know within the context of the story. Sherlock Holmes' sister shouldn't know in advance the outcome of the Second Boer War. Similarly, a muggle character in a Harry Potter fanfiction shouldn't know all about Hogwarts before he's met his first witch or wizard. The books make it clear that people with magical abilities work very, very hard to keep their world secret, so your muggle OC shouldn't be aware of them at the start of the story.

Finally, make sure your character has opinions and prejudiced suited to the society they live in: Klingons should value honour; Tolkien dwarves should value gold. Your character can probably get away with one weird, counter-cultural opinion, especially if you explain it well, but probably not a raft of them.

1) Essentially, they're a bunch of pointy-eared space Stoics.

2) A debilitating disease of elderly Vulcans that leads to a loss of emotional control. Not as much fun as the 'medical problem' I'm sure you all thought I was going to mention, Pon Farr. I mean, we all remember when Spock went crazy and threw that soup at Nurse Chapel, right?