When shooting somebody in a dark auditorium I usually use a 250 watt Lowel Pro light with barn-doors placed about 10 feet off to the side, and I use an inexpensive 600 watt dimmer to adjust the intensity. This is a very small and portable package:
Lowel 250 watt Pro-Light Focus Flood Light : $130
Lowel Complete Four Leaf Barndoor Set : $33
Lowel O1-33 Omni Light Stand - 9' : $68
Impact 600 Watt AC Dimmer Control : $25
...If you can't place the light close to where you have to film, then you are in to a much bigger light like a 650 watt fresnel or an optical spot with a much bigger stand to support it:
Arri 650 Watt Plus Tungsten Fresnel : $378
Dedolight DLH650 650 Watt Stand Mount Spotlight : $960
If you are unsure what you need, then see if you can rent a couple of lights to try out. ( I own 6 Lowel lights, which includes 3 Pro lights with dimmers. They are compact and very handy to have around. )
