The Past Continuous tells us that the action started but had not finished (it was ongoing) at a specific point in the past. It may tell us the action was simply ongoing, but it may also tell us when the action was interrupted. I was walking home at 3pm. (The action was ongoing at that time – it started before 3pm.) They were eating dinner when the phone rang. (The ongoing action was interrupted – it started before the phone rang.)
In storytelling, the past continuous may be used for background information, or to present an action or event that came before described events. This is generally done by showing a past simple event (often in a time clause) interrupting a background event in the past continuous.
- He was taking a bath when the police knocked on the door.
- She was riding the bus when she first saw him.