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This is actually the wrong sub-forum to be posting a topic like this, this place is for introductions. You want to repost this in the General Discussion Forum, you'll have much better reception. Also by "Movie" I must assume you mean; a Machinima, which is much more difficult to make then most may realize. You need a good HD CC, if you want to submit it to Machinima and a good editor to put your video together. If you are putting out a casting call make sure you have the basics already, "Good Director" for managing scenes and figuring the order that they will be filmed in, a "skilled editor" otherwise its just another gameplay upload, and "a talented writer" with just one more guy handling the sound files that you recorded from Skype or whatever you used. The art of making a film is what its all about, and on a side note you can't sell any movie you make using Halo. It would have to be a fan-video uploaded by you and your friends to avoid any copyright infringement.
If this is a "real" on location with set and actor's video then good luck. 343iindustries wouldn't really allow that to happen on the legal scale unless it is "a fan video" or a "parody" You still need all the same things that you needed from the Machinima only this time you need money, and lots of it if you want anything half way decent. Give people a demo and synopsis before you casting call them. Also, try and avoid posting a topic that would call for people to meet up with you in real life, this is frowned upon and is quite unsafe. Take all the nesscarry precautions, have people provide ID and provide some ID yourself.
Anyway, welcome once again to our awesome forums, watch yourself out there and have fun.
Z