Both the past simple and the past continuous refer to completed actions in the past.
Most of the time when we are talking about such actions, we use the past simple. This is by far the most common way of talking about the past.
Only use the past continuous when you want to emphasize the continuity of the action.
When we use these two forms in the same sentence, we use the past continuous to talk about the "background action" and the past simple to talk about the shorter completed action.