Usually, the goal is to have rod leg of the loop to unroll parallel to the ground or water (that can change depending on the distance to the target, wind, and other variables). And to have the fly leg of the loop parallel to the rod leg. There are several variables that go into making this happen, but if the legs get out of parallel bad things such as tailing loops or flies crashing into the water happen.
Here's a video by Lasse Karlsson. He is an IFFF master casting instructor from Europe. In the video he is adjusting where and how he stops during the cast. He demo' s good and bad stops. Note how the loop shape and direction is impacted by the changes.
Remember, good is everything parallel. https://vimeo.com/38456753
Concerning the "River Runs Through It". I strongly recommend ignoring the "casting lesson" at the beginning of the movie that uses a metronome and a book under the arm. Yes, it does work, IF you only want to cast a short distance. But if you ever want to cast long distances then you will have to unlearn the bad technique of that casting style.
Good luck and listen to Robin's suggestion. Clubs are a wonderful place to learn.