To do this the old fashioned way, you need a single - very sharp focused source. The key factor is a single point source - which means you also need a bright light.
You can use (in order of effectiveness, which is pretty much based on brightness):
1) A moving head profile fixture (ex: Mac III, VL3500, VL1000). These fixtures are similar in concept to a leko mentioned below in that they can focus the light sharp enough to project patterns, or in your case - sharp enough to make a perfect silhouette. Only downside is most of these fixtures are 30 degrees or narrower (Mac III below is 55 degrees, but very expensive). This means you will need quite a bit of depth to achieve even coverage of a square/rectangle screen.
2) An ellipsoidal reflector spotlight or "leko" (ex: ETC Source 4). These fixtures can focus the light sharp enough to project patterns, or in your case - sharp enough to make a perfect silhouette. The 750w lamped Source 4 is pretty punchy, however renting a k5600 400w or 800w Joker with the Source 4 adapter makes it even better. The nice thing about these is that you can usually find 50 degree fixtures in any theater, reducing the depth you need. There are 70 and 90 degree lenses now, but I would questions the evenness of the beam having never used them.
3) An open face fixture with black reflector installed (ex: Arri X series). These fixtures have a single lamp with a simple reflector behind it. The concept is the same as a Lowel Tota, Cyc lights, etc except usually with a different style lamp like a fresnel lamp or HMI lamp that is not as long. Many of these fixtures have the option to change the silver reflector to a black reflector - eliminating multiple points of light and making the beam very sharp and distinct. One specific fixture I know of is actually sold with the black reflector and called a "silhouette light". You can also cheat this effect by using a cyc light and either painting the reflector with high temp black or just covering it with blackwrap. A cyc light doesn't work quite as well since the filament of the average 1000w cyc is about 3" long, where the single ended lamps on their side have a square filament around 1". The upside is that these fixtures are extremely wide (100-130degrees), although the light will fall off on the edges - which can be very apparent with a large area. The other fixtures above will be even across the whole area, but require much more depth. The downside to these is really the brightness. With the black reflector, the efficiency of the light does way down since much of the lamps output (back 50% and some of the sides) is wasted. HMI fixtures like the Arri X Series aren't too bad due to the bright source, but a small or tungsten version might not do too well.
The other thing you need to think about is your screen. If the theater screen is a white front projection, it's probably useless for this. If it's a grey rear projection - that's perfect. You'll have to be very careful with your light placement in order to not see it through the screen. A thicker screen will hide the source better - if you can get your hands on a "low-gain" screen, they are usually very thick and block the source very well. The other option is to raise the screen (and actors some) above the light level, but then you may have perspective issues. I have found shooting through a white fabric to be a bad idea, you see the light source quite easily.