FORM - was/were + present participle

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1. Interrupted Action in the Past
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·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.
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Examples:
-I was watching TV when she called.
-When the phone rang, she was writing a letter.
-While we were having the picnic, it started to rain.
-What were you doing when the earthquake started? -

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·In USE 1, described above, the Past Continuous is interrupted by a shorter action in the Simple Past. However, you can also use a specific time as an interruption.

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Examples:
-Last night at 6 PM, I was eating dinner.
-At midnight, we were still driving through the desert.
-Yesterday at this time, I was sitting at my desk at work.

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credits:

http://www.englishpage.com/verbpage/pastcontinuous.html