So, you've booked an event and purchased all of the necessary equipment. Now let's move into the auditorium and get ready to shoot. After conferring with the producer of the event (very important to keep in contact with the producer—it maintains your credibility and keeps the peace), try to find a vantage point dead center in the auditorium. Most stage events are designed for a narrow group of people sitting in the center of the venue, so why not grab the visuals from that spot? Plus, angled shots necessitate constant re-focusing from one end of the stage to the other and increase the possibility of "seeing" into the wings and possibly recording something that shouldn't be seen.
In 99% of the venues that I shoot (ranging from school all-purpose rooms to conventional theaters with seating for more than 3,500), I set up and shoot from the rear—the last row, the walk area behind the last row, or the balcony. With a 97.5mm lens (Canon 13x) I can get as close a shot as I need to, so distance is not a problem. When picking a spot, keep fire codes in mind. You cannot impede movement or block aisles or exits. If in doubt, ask. Most events of this type require a fire marshal or firefighter to be on the premises; by asking them where you may or may not set up, you will save yourself the hassle of having to strike your equipment and move it to a "legal" location. Also be sure to secure all cables (you don't need a lawsuit).
Where to place microphones on a stage is a science all in itself. A lot depends on the type of event, number of performers, area to be covered, etc. For most plays and general stage performances, I'll set 2-4 condensers on stands along the edge of the stage (on the auditorium floor), with the head of the mic just slightly above the stage floor at a 30-45 degree angle, then plug them into an audio snake that feeds into the mixer. Whatever an event's audio needs, remember to make every attempt to mic as close as possible. This minimizes conflicting sounds (audience chatter) and echoes. I also have had good experiences with wireless equipment (both handheld and lavaliere), but for these uses—particularly with handheld mics—you may have to adjust the sensitivity (if possible) to increase the pickup distance. You also may want to position a microphone to pick up audience reaction. Just remember to keep the level low enough to prevent stage pickup but high enough to pick up applause and other audience reaction. Careful adjustment here will save you tons of adjustment in post.
Another method of acquiring audio is via the venue's sound system, if available. Many producers will avoid tying into a sound system unless it's staffed by a professional sound person. I will do the opposite. I try to gain access to the soundboard whenever and wherever I can. I make sure that I determine what levels are set and how I am patched into the board. If you intend to tie directly into the board, use a line-level or XLR output and an audio isolator to prevent ground loops. I prefer the wireless method, using an "instrument" transmitter, like those used by rock guitarists. Their design is perfect for the line-level tie-in points of a mixer. If the house system tie-in doesn't work, then you can fall back on your own mics. Remember, audio is 50% of a stage production (and maybe more in a music-related event).
Lighting also is an important part of any production. Many theatrical performances use some pretty extreme lighting, from almost total darkness to daylight-like brightness to the ever-pesky follow spotlights. Talk to the lighting designer or director to find out what the extremes are, and attend a rehearsal or have the lighting crew run through the lighting sets so you can judge the lighting for yourself. I find that running a slight gain and full manual iris is the best bet.
Color balancing for theater is tough, even for seasoned pros. Few theaters use uniform lighting, meaning that halogen, incandescent, halide, and other more exotic lighting techniques are used in combination. All have different color temperatures; you could start off being perfectly color-balanced and end up looking blue or yellow as the performance progresses. You could try auto color-balance and plan on correcting the "bad" scenes in post. Inquire about the type of lighting being used, although if it's a school auditorium, there's a good chance whoever you ask won't know. Older schools probably are using incandescent, while newer or refurbished auditoriums are probably using halogen, which require filters of 3200 K and 5600 K, respectively. Just watch out for the follow spot. Ask if they intend to use one. If so, ask if they could eliminate it on the night of shooting or use a slight yellow filter (when used with incandescent lighting) to help balance the color with the stage lighting. Otherwise, you will turn spotlighted actors blue.
Now, you are ready to shoot your first stage performance. Just remember to use common sense and standard video- and audio-recording procedures. Watch your production (before editing) to critique what you have recorded. The best way to learn is to learn from your mistakes. It also doesn't hurt to ask for help from videographers who have a few of these under their belts.