Originally Posted by A_Dude
1 - How do you find out what was done to achieve the aspect ratio used (in this case, open matte) ?
Some films are shot in a 4:3 format with a window for the intended theatrical presentation AR in the camera's view finder, to help the camera operator frame the scene as specified by the director. This is in anticipation of "fullscreen" DVD releases, etc. There is more picture recorded for each frame above and below what you see in the theater. For discs and television presentation, they just leave more of the picture in.
Say that it was shown in 2.35:1 AR in the theater; if shown OAR on your screen, there'd be black bars above and below the frame. If they actually shot it in another AR, they can essentially remove those black bars--nothing is lost, just more of the frame that was shot gets revealed. It's more than the director intended that you see, but that's not something that will bother most viewers (definitely not the way that showing a 16x9-shaped portion of the wider AR does).
You can see a Wikipedia article on open matte here