Example situation:
I am a native english speaker, move to a foreign country, marry a girl from that country who knows not one word of english.
She studies basic conversation english for a year or two and from there we start watching loads of english movies all the time and I just explain new idioms and grammer from time to time. Think its possible for her to become pretty fluent that way? With just me and tons of media.
Actually I work as an english teacher in Japan. Not as a college professor but I do have lots of experience teaching all kinds of levels.

Yes, it does work. I have learnt Italian and English from TV. It also requires a dictionary, because at first it is really difficult understanding even basic notions. But after a few months, it is very easy. It helps a lot reading books you like in the language you want to learn, and talking to people waho speak natively the language you want to learn, but it is important that they don't speak your language.

yes it is a very good method

No. People in movies speaks for fluent speakers so they don't speak clear and mumble alots.

Not really the best. Especially on elementary stages. Like you can understand nothing. It can improve pronunciation, I believe. Likewise you voice becomes better when you listen to somebody's good singing. Your vocal chords just vibrate in accord with what you hear. Though, most films contain up-to-date frequently used vocabulary. The best way to learn a language is speaking and writing. Still films is better than some others.

I think listen, speak and write more are essential in improving a language.

Um... if you said that the girl didn't know one word of English, I think she better takes some basic courses or employ a private tutor because learning a language is not an easy task. Every language is hard for a beginner as every single languages also got their own grammer & structure. Besides, you are gd in this language doesn't mean that you are a gd teacher and know how to teach her.

Also, if you select some movies which are talking about some complicated issues, how can she understand?! She will just lose the interest of learning it.....

Yes that is the best way to learn a new language.... books can only doo the theory part ... the only way to learn to speak is be listening to or bieng around ppl who speak the language and ofcourse trying to speak it as much as possible... i mean thats how u learnt u'r language when u were a kid

You never know, it could be just slang, and not formal at all!

Fluency is being able to open your mouth and let it flow. By watching these movies, your student is acquiring simply a feel for intonation and a passive knowledge of some aspects of the language. I think it would be better to sing, if you can arrange that. It would get the muscles working and it would enable your student to retain the vocabulary in her mind. It would also give her confidence to open her mouth and say things. Put together a little repertoire of songs and see how it goes.