"Haunted House" shooting the actors
I shoot a LOT of theatrical photography and I can tell you that haunted houses are probably gonna be a very tough assignment. As has been said unless the gore and costumes are really impressive even a properly exposed image is just going to be a properly exposed image of poor effects. You can't help that, of course, but it'll be important to set clear expectations with the show-runner.
Here's what you can do, though.
Don't try to cover the entire house. Instead, pick one to three moments where the makeup, costumes, and set are the best without the lights and focus your attention on making these look great. Pose your actors at the height of drama.
If you have access to the light board that's best -- you can boost levels on the lights themselves, perhaps even set strobes to continuous (if possible; if not, just take a lot of shots until you get lucky).
Another option is to gel your speedlites and put them wherever the lighting is in the room -- essentially, you'll be "boosting" it. You can even go further and light the scene yourself as you think best. It depends on your mandate -- if the goal is "record the haunted house as guests see it" then you are stuck within a narrow range around the lighting and blocking choices they've made. If your goal is "make people come to this show" then feel free to move people, lights, and elements around as you see fit.
Here's a before-and-after to consider. The first image is the lighting as it was set in the show. The second is after we played with the lights to create an image that actually looked like the show the audience experienced.
If I hadn't had the lighting designer on hand, I would have popped a speedlight up in the basket or, more likely, simply wouldn't have attempted this moment (which would have been a shame!).
Obviously on top of all this you want a good camera with a great lens on a stable platform. I shoot most of my stuff at 3.2 on a Canon 5Dmkii and a 24-70 2.8 L on either a monopod or tripod (depending on how much I value mobility vs. stability). In a pinch, the Canon or Nikon "Nifty Fifty" 50mm lens, which goes all the way to 1.8, is a good choice, but you'll need a decently-sized room to use it, particularly if you're using a crop sensor. I used the nifty fifty for a very long time and I strongly recommend it for all photographers -- in terms of bang for your buck, it's the best $100 you will ever spend on a lens, hands down.
And yes, SHOOT MANUAL AND IN RAW. TTL is gonna be totally flummoxed by the weirdo lighting going on and won't know how to set any exposure, let alone flash power. It's not too hard to set the power on a flash and your exposure settings on your camera. If you aren't up to doing that, you should pass on the job because you'll probably just get frustrated and make your friend mad at you.
Good luck, and let us know how it goes!