English is not my first language, so I have difficulty sometimes grasping the nuances of the language. Like for example the difference between lie and lay. My understating from reading about it is that we say "I want to lie down on the couch" because I lie myself but lay something or someone else. But, the other day I was watching a movie, and in a scene an actress said "I want to lay down on the couch". So, which one is correct? I mean is it possible that people in the movies use it incorrectly, or my understanding is not accurate?