I believe that your statement is false (at least based on my personal observation). When you learn a word from a movie, you learn understanding its meaning. In order to use the word you need to know the context in which it can be used and how it fits these contexts. In my view learning words by watching a movie won't help much with talking or writing. This may differ for different words (e.g. the context for nouns is simple, but for conjunctions it is much trickier). Do you see what I mean? Or did you mean some other way of "learning" words while watching a movie?