Use the Past Continuous to indicate that a longer action in the past was interrupted. The interruption is usually a shorter action in the Simple Past. Remember this can be a real interruption or just an interruption in time.
The Past Continuous is used to talk about what was happening at a special time in the past. It is also know as the Past Progressive. You will be able to see the forms, the use and there is also a link to test your knowledge of the Past Continuous.