We have found it useful to clearly demarcate actor actions from system responses. This allows us to add more or less detail to either side of the conversation, as you'll see shortly. There is also a list of alternatives to the main course of the use case. These alternatives represent a reasonably complete list of conditions that system designers must be able to detect and to design appropriate responses for. Insufficient funds in customer account.
User wishes to perform an additional financial transaction.
Use cases should be constructed by business domain knowledgeable people. Use cases should be written for both audiences. Actor interactions and system responses need to be described at a fairly high level. On the other hand, descriptions of system responses must contain sufficient detail so that object designers working with analysts can design a reasonably detailed object model. Walking this fine line between sufficient detail and bogging down in details requires practice and critique.