'I have just watched the movie' An action starting in the near past and extending to witness completion in the present. "I had watched both movies before he arrived late night yesterday." Here the completion of the action is a past one ,that took place before another action in the past. 'had watched' could be also used as a conditional tense specially in 'if clauses' as in : "If you had watched the movie's trailer yesterday, you would have decided to go seeing it with me tonight." It indicates a hypothetical situation ( if you had watched, in fact you didn't ) in this case.

maybe an example can work to explain this: - I have watched the movie. - I had watched the movie before it was promoted in my school. In the second sentence, the event of "you watching" happened before "film promotion". "film promotion" happened in the past and "you watching" happened even before that.

I have watched.....................A period of time from the past up to now. I had watched......................A period of time from far the past to the past.

maybe an example can work to explain this: - I have watched the movie. - I had watched the movie before it was promoted in my school. In the second sentence, the event of "you watching" happened before "film promotion". "film promotion" happened in the past and "you watching" happened even before that.