Certain methodologies suggest that you identify system Actors early on in the process of defining your problem and solution.
I think this is a great idea, and this is why.
Identifying the Actors early on in you process helps define the boundaries of the business context. By knowing the ‘Who’ you have in effect defined “Who Isn’t.”
You have also effectively identified your SMEs for your business and system requirements. They are going to be people who fall into one or more of the real-world roles (or manager’s for those roles) represented by the Actors you have defined.
For each Actor, you can define the actions they will perform in the system and you have identified the candidate set of use cases to be detailed as part of you requirements effort.
And don’t forget, Actors can be other systems that will interact with your system, so you have also begun to identify the functional/data integration points that will feed certain non-functional requirements.