Let me provide an answer from a bit different angle from the other two.
This sentence pattern 〜たら〜ていた
is used when you say that you found something in a particular state when you did something. It must be describing some kind of state, rather than a one-time action, because the verb of the main clause, やる in your example, is used in the ている form. Unless you happen to find yourself in a state of unconsciously doing something with a movie the moment you turn on the TV, this verb must be referring to an act by someone else.
So, your first interpretation is the correct one.