A Wizard of Oz: acting out the systems functionality, can help when a system has not been build yet, but a realistic user test is necessary to drive design.
In a Wizard of Oz study the user is made to believe he works with the real system, while in practice someone ‘behind the screen’ controls the system behaviours. Sometimes temporary software is build to avoid the user detecting the simulation.
The Wizard of Oz technique is popular in Service Design projects, although the requirement that the user should not know he is not using the real system is sometimes dropped. This brings the technique close to role-play though.