Sometimes your setting isn’t yours.
When you’re writing in a shared universe, you, as the author, are coming into a setting where someone else has already done some of the designing.
This might be franchise work (consider a setting such as the Star Wars Expanded Universe, or a novel series based on a video game or role-playing game), or perhaps you’re just role-playing for fun. The setting that you are writing in has already been outlined. Your job, as a storyteller, is to create a novel or short story or character that fits into that universe. This involves reflecting not merely the setting, but also “flavour,” themes, and overall experience.
If you’re writing in a shared setting, my number one advice is to respect the rules of the setting. If the setting states that human beings each have one and only one magical talent, or there are no aliens in outer space—please follow those rules. Your characters don’t have to defy the rules of the setting to be interesting. The franchise owners are looking for people who can tell compelling stories within their pre-existing setting. They want characters who are complex and interesting and who fit within the rules—characters who embody the kinds of people who can live in the world of the video game, or role-playing game, or movie universe, or whatever kind of world it might be.
Sometimes it seems as though beginning writers think the best way to make their characters interesting is to break the rules of the setting—to be the first known alien in the no-alien galaxy, or to have three magical talents in a setting where most people have only one. What makes a compelling character isn’t what their character can do, it’s the kind of person their character is, and the ways in which that character interacts with other people and the world around her/him. Similarly, a powerful, rule-breaking artifact doesn’t, in itself, make for an interesting story. Readers are looking for a tale that conveys the flavour of the setting, not a story intent on breaking the setting to suit itself.
If you’re writing in a shared setting, please read the lore before you begin outlining your story, and ask your editor, lore keeper, game master or overseer if you have any questions. It’s much easier to ask about an idea first, rather than have to make major edits to a completed novel because you assumed that all sci fi would have aliens, or forgot that in this setting there are no aliens in outer space. Suddenly you’ve got a book about human-alien relations that doesn’t fit in the setting you’ve been contracted to write about.
Shared settings work because of common rules accepted by everyone who’s creating works within that setting. If you’re hired to work for a shared setting, it’s because the people who hired you want stories that reflect the world of that IP (intellectual property) or setting. Your job is to reflect the world of the shared setting, as creatively as you can, while providing an experience of that setting that its audience is seeking.