How one could obtain relevant information for the development of the case model. What is the correct approach?
@Ister is right : capture customer's needs correctly is a whole topic and a complex one.
There are some hints which can be provided:
If you get a state of work, this is the first source
If you get a real customer, after the state of work, workshops about his business and his business processes is the a source
If you want to realize a software with competitors, looking to competitors products is a way to find use cases
State of the art of the domain is also a source
Interview with actors of the domain may be a source
If you work in a domain such as transport, army, medical; there are some norms which can be used to define use cases
Doing UI mock up could be a way to define use cases and business processes. Espacially with non "it aware" guys. The UI is done to realize the business processes, so to show ui mock up may help some customers to define their needs.
your own experience