into its role of AgeCalculator and have it play its part. In this context, it is like an actor from Candid Camera. The poor ClientDetailsValidator has no idea it's being duped.
Verify State When Appropriate
Some very experienced developers remain unconvinced about verifying interactions using mocks. Their position is - how an object talks to other objects is its own business and shouldn't be explicitly defined in a test method. Instead, the test should verify an object's state (or data) to ensure something happened.
I tend to use a mixture of both, and again the behavior vocabulary helps. For behavior that is best described in terms of interactions, I'll use mocks. For behavior that produces an output (say a calculation) I'll verify the result.
In general, if you look at a system as being made up of cooperating components (aka services or subsystems), then for the classes within an individual component I tend to verify interactions, and at the "edges" of a component or service--when there is some tangible effect on the outside world or a message being passed around--I will verify state.