For retrieval to occur, the appropriate retrieval cues must be present.  The retrieval cue is an external (or sometimes internal) event that initiates the memory retrieval process of ecphory.  A retrieval cue could be something like the question "describe what what you saw", but can also be a physical stimulus (e.g., pictures, sounds, smells, etc.).   For example, if you were called as a witness to a traffic accident, you might be asked to describe what you witnessed.   Note that is possible for an external cue to serve BOTH as a retrieval cue and as an interpolated event.