How to send Interface rule input value to the Expression Rule?

Some concepts will help you with the way data is passed around between objects in Appian:

1. a rule input (on an Interface or an Expression Rule) is a reference to a location where the value is actually held. In Java terms you can think of a rule input as a "pointer" to a location in memory. Crucially, a rule input is not a value, but a reference to a value
2. If an Interface has a rule input then the value will be typically be in an activity class variable in the the User Input Task node in the process that is displaying the Interface 
3. to "send an Interface rule input to the Expression Rule" you'd map the Interface Rule Input to the corresponding Expression Rule Input, something like: myExpressionRuleInput: myInterfaceRuleInput

In addition to Stewart's questions above, I have some slightly simpler preliminary questions:

1. What are you trying to accomplish?
2. What have you already tried, and in what way does this seem to not work?