past simple
for completed actions in the past. The ship left an hour ago.
for actions which happened immediately one after the other in the past. He stood up got his bag and left.
Past Continuous
for actions which were happening at a specific time in the other in the past.
They were watching TV at 7:00 yesterday afternoon.
for actions happening at the same time in the past. She was making dinner while he was washing the dishes.
When one past action interrupts another past action we use the past continuous for the action which was happening and the past simple for the action that interrupted it.
He was reading his paper when the phone rang.
The phone rang while he was reading his paper.