Usage Domain Analysis
The analysis of the usage domain is about how somebody/something, a person or another system, interacts with the system-to-be. The two suggested activities Use Case Candidates and Actor Candidates may be parallel processes. In some instances, there are obvious use cases; but at other times, there may not be an immediate obvious candidate. In such instances, an analysis of the possible actors may reveal use cases. The actor analysis may be performed in parallel with or after the Interface Analysis, especially the interface with other systems.
So jumping back and forth between several activities should not deter the developer from searching for all possible candidates, be it actors or use cases.