I've given a fair number of speeches in front of groups, many times to people with a pretty good knowledge on the subject, though nothing on the scale of your audience. One thing I realized early on and have seen in a fair number of others, is not the fear of speaking, but the fear of being in front of and watched by large numbers.
One thing I found that does help, when appropriate, is not to stand in front of the audience. Stand to the side, behind or even among them. Focus their attention on the briefing media...the big screen in your case. Perhaps a 4-5 minute trailer with pictures of your favorite moments while filming, shots that didn't make the film, or examples the film itself will expound upon. IMHO people tend to be far more visual creatures and appreciate pictures more than words, give them what they want.
When the film is complete and your audience is on their feet. Take a humble bow and enjoy the applause.
Cheers