The cover story establishes the context of a GBS. The cover story defines the premise under which the student pursues the mission. It lays out the scenes where the action takes place, and other details that flesh out the GBS, making it more plausible and enticing to the student. The cover story is crucial in drawing the student into the task from the beginning. A boring cover story will lose the student for the entire GBS. It's just like reading a novel. If in the beginning the book sounds like it won't be fun to read, you probably won't want to keep reading to see if it gets better.
When designing a cover story, it is important to make the story coherent and realistic. The elements of the cover story should be thematically consistent with each other to avoid frustrating the student. If a given role would ordinarily include certain tools, and certain locales, the simulated setup should provide those tools, and the scenes of the GBS should include that locale. If a GBS is to tap into students' experiences, it must reproduce those aspects of the cover story which will make it real to them. If students pursue an implausible cover story, they will not be able to use common-sense knowledge they learn when they try to solve "similar" problems in the real world.
The only place in which it may be desirable to reduce realism is when providing teaching support to students. Once students assume the cover story, they should be provided with the support needed to do the job. When possible, it is important to provide help through materials that are consistent with the cover story. However, sometimes it is necessary to go outside the cover story to provide needed support. When teaching support simply cannot be smoothly integrated into the cover story, the support should be provided anyway.
The Focus of a GBS
Where am I in the content of the book?