A Civilian is someone who is not a member of a military or paramilitary organization, and within the Federation. The task of a Civilian varies from being a bartender to a teacher, scientist or even a reporter. Although this post seems unimportant, it is actually quite necessary for Starfleet officers to function smoothly and efficiently.
Simming this post is more difficult than others because there is little known about this position as compared to those of other duty posts. However, this leads to more creativity in the part of those simming this position. This creativity shows through, most noticeably, in the way a civilian can approach missions and other plot driven situations. Was the civilian in the wrong place at the wrong time leading to situations that they have no training or desire to be in. Do they have a background and skill set that allows them to help save the day even if that means treading outside the protocols and procedures Star Fleet officers are bound by.
Star Fleet officers are expected to be the ones that to go boldly into the unknown and they quite regularly risk their lives to solve the unsolvable. How would a merchant survive an away mission gone horribly wrong, or how would a botanist deal with being in charge of a first contact situation? These aren't situations that these people are trained for, nor do they expect to find themselves in. Just regular people caught up in extraordinary events. As said previously, it can be difficult but it allows for a great deal of character development and by writing a civilian you will further bring the environment you sim in to life.