Can watching Supernatural develop my English

It helps to watch movies of the language you are learning. first watch it without captions and observe their actions and get a feel of what their words are trying to convey. Try to imitate their lip movements even if you don't understand what they are saying. This help loosen the muscles of your mouth and familiarize you with how to form the words. Then repeat watching the movie with the subtitles on. I would also suggest you learn and sing the songs of the foreign language you are learning.

Some people learn by just watching videos of the language they want to learn. It depends on your brain :D

Using caption and speaking he captions as the show plays will help you learn English. Plus, 80's slang.

Whatever works best to improve your English. You should repeat out loud the English you hear.

are you American?where are you from English country?