Your biggest challenge to shooting during the show is that the audience will be really dark. The thing about renting camera bodies is that they are expensive, and it takes time to get used to the features. If you are used to a 5 year old entry level camera, it's going to take you a decent chunk of time (days or weeks) to get used to the 5d enough to reliably shoot. Now, knowing that this is just as a favor for a friend, maybe that's not that big a deal, but since you'd be paying money to rent a camera, you'd want the shots to be great. If I were in your situation, with your equipment, I would look at lenses rather than a body. I'd simply rent an 85mm 1.4 for reaction shots of your audience, and I'd rent a 15mm 2.8, and get permission to get as close to the screen as possible.
You have a camera body, and the odds are that you aren't going to go out and drop $7000 on a 1dx when you currently have an older entry level body. My advice is to use what you already have, rent a couple of lenses, and figure out how to take great shots with your equipment. Yes, you can get a 5d3 or a 1dx, and get great shots, but the next time you're in a difficult situation, you'll want to go out again and drop a few hundred dollars on renting a camera body too. You can do these shots with your 450d.
Your main limitation is that you're not going to be able to get a shot of the screen showing what's going on while ALSO showing reactions from the audience. Since this is just for fun, and to help a friend, I would recommend trying things you don't normally do, it's a license to play!
If it were me, I'd be looking at these shots:
- People reacting positively to the show, laughing, smiling, crying, especially two people together with a similar reaction.
- Shots from the back of the place with the screen exposed well, and rows of peoples heads
- Wide angle shots from close to the screen that are exposed for the screen. (the rows of people will be pitch black)
- Wide angle shots with the screen AND some of the people in the front rows. (Same perspective as before) Later, you can composite shots of people watching with shots of things on the screen.
- Any sort of detail shots that are pertinent to the situation. Tickets, popcorn, heads on the shoulder of their date, etc.
- And most importantly, talk to your friend, and figure out what shots he's hoping for and expecting.