For those who have - how was your immersion experience ? For how long did you attend ? Is your French better ?
Since you're taking private classes I imagine that you will be making more progress than in a group setting. So don't worry about my comment on 3+ weeks being best. Whatever time you decide on is better than nothing. Brussels is an interesting city, I hope you have fun!
Regarding French movies: I'm not much of a movie person and for various other reasons don't use it much to learn a language. I'm sure it's helpful, especially if you can turn on French subtitles in the beginning.
I used to listen to a lot of podcasts, which helped. The obvious disadvantage is that you don't have any action on screen that can help you understand what's going on if you didn't understand the French. I also like to read, but that can also be difficult at the beginning. If you buy books in Brussels, try not to buy books that will be too difficult to read and discourage you. That doesn't mean that you should buy something you're not interested just because it's easier; it may be a good idea to ask your French teacher which writers she can recommend for a genre you like that aren't too difficult to read. Or buy a book you already know in its French translation.
Regarding working in foreign countries: I only partially agree with Robert. I've worked in Brussels and Geneva and didn't make any progress at all in Brussels, and in Geneva only while I was living at a French woman's house. Unfortunately, my colleagues in both places either couldn't or didn't want to speak French. It doesn't matter so much WHAT you do in the country, as long as you do it in your target language.
I've done several courses abroad, including in France. I've found that I benefited the most after a stay of three weeks or more. Two weeks was just enough to settle in, but too short for solid improvement. Nearly all of those stays helped me improve my language, provided that there was actual immersion. What I mean is: don't switch to a language other than French at home or in school (people tend to do that during breaks when the teachers are out). That can be difficult when other people want to speak English because they find it easier, or if they're fellow Americans and you would feel strange not to talk to them in English. You will benefit the most if you avoid falling into that trap.
Mouy seems to be a very small town. Is there a particular reason why you want to go there? You might get bored in such a small place, but I suppose it could also help you focus more on learning.
Immersion is always best. I am a French teacher. Back in my college days, I did a summer program at the university of Lyon.
My best immersion experience was actually working in foreign countries. I never would have become fluent in Polish had I not worked there.
You say 3 weeks or more is better ?! I wish i could spend a month or so but i cannot affordable to do so. I will be back packing in Brussels for a few days on my own after the immersion to see how i get on in French by myself .
One more thing .. did watching French movies help you learn ? What other methods did you use ?
Merci beaucoup !