A high-level project definition and preliminary idea about scope is needed before use-case analysis and prototyping exercises can be effective. Most developers are detail oriented and will consider this high-level definition too vague to be useful. Keep in mind that the purpose of this high-level project definition is only to determine scope for the use-case analysis (not for coding).
Here is an example of a high-level project definition statement: Build a system that assists project managers in planning tasks, tracking the activities of every team member, and estimating completion dates. The project-tracking application should allow the user to do the following:
Define a project and its tasks.
Record people assigned to project tasks and estimate the time needed to complete each task.
Record the order in which tasks will be completed.
Record project progress and mark tasks as completed.
Automatically schedule the project.
Create reports of project progress.
As vague and simplistic as the statement is, it provides a starting point for identifying actors and constructing use cases.