Here, 'had been watching' indicates Past Perfect Continuous. It
is used to indicate an action in the past that began before a certain point in
the past and continued up until that time.
Past Perfect (had watched) is
used to indicate a completed action of the past that happened before another event
took place.
Future Perfect (will have watched) is
used to indicate an action that will be complete before another event takes
place.
Past Continuous (was watching) is
used to indicate uncompleted action in the past (with or without time
reference).