When you create requirements, it is important to develop the requirements in multiple stages. For example, you may want to do a general layout of the program and create higher level use cases in the first session to get a feel for the overall requirements. In the next session(s), you can focus on each key feature to ensure that the normal paths are all defined in the use cases and further refine the prototypes. In the next session(s), you can attempt to define all of the error conditions which can occur and update the prototypes as necessary. The final sessions should review all work previously done to ensure that all requirements are clear and complete. At each stage, you should not be afraid to revise work done in a previous step because getting the requirements correct will ultimately save time in the more costly development and testing stages.