Now, it is 7:55. You will watch a movie at 8:00. You are about to watch a movie. 5 hours ago, it was 2:55. You planned to start watching a movie at 3:00. However, the phone rang at 2:58. You were about to watch a movie when the phone rang.

The difference is the tense "was" and "am". If I am about to do something, I plan on doing it very soon. If I was about to, it means I intended to until something happened that changed this. For example: I am about to go to the store. I just need to make the grocery list. I was about to go to the store. But I forgot an appointment I had, so I'll go tomorrow. Using "was" means you are talking about your past self, and now you plan to do something else. For your example, I am about to watch a movie. Come join me! I was about to watch a movie, but then the electricity went out. Hope this helps!

I am about to watch a movie: In a moment in the future, you WILL watch the movie I was about to watch a movie: A moment ago in the past, you WERE going to watch a movie (but didn't)

I am about you will do it But I was about you were going to do it but you didn't