Difference between Past S & C
The ‘action’ described by the past simple tense interrupts the ‘situation’ described by the past continuous tense.
Examples:
I broke my leg when I was skiing.
I was playing a computer game when the doorbell rang.
the past continuous describes ‘situations’ that go on for some time – ‘skiing’ and ‘playing’ but the past simple describes ‘actions’ that happen quickly – ‘broke’ and ‘rang’.