Tackling this question raised by my blogpost during the Titanic centennial, on the four corners of a wreck: What's it like to participate in a History production?
The short answers: Fun! Long days!
I've appeared in a dozen-odd shows for History, CBS, and National Geographic, but the production Titanic at 100 that aired last year on the day of the wreck's centennial had a bigger budget (since it was part of an international marketing push for the network) so I'll concentrate on that one. The production company was Lone Wolf Documentary Group of South Portland, Maine.
My bit started in June 2011, when Lone Wolf flew me out to their location for a day of interior shots that would serve as a screen test for History's executive on the project (called the showrunner), and, if acceptable, would be part of the show.
The cameras at that setup were digital single-lens-reflex cameras, aka DSLRs, which produce broadcast-quality HD video. The setup also included pro-quality sound gear recording on a separate chip, professional lighting and a miniature dolly to move the camera across the room.
The equipment filled the back of a van and to set it up all the way to mic checks required more than an hour. I wore a lavalier mic and wireless set, and there was a sound man wielding a boom and watching the levels. Audio quality is very important!
That was one shooting day.
After History okayed me to be a participant in the virtual hangar segment (along with long-time Titanic experts Ken Marschall and Parks Stephenson), in October I flew back to Maine for two more shooting days. The setups the first day were two more interior shots (one against a green-screen, one at a desk by a window in an old building); an exterior shot where I walked along the harborfront; and an exterior shot as I drove around Portland.
One thing Lone Wolf is good about is feeding the cast! There must be some ex-Navy submariners on the staff. We commentators met one or both directors for breakfast before a shooting day started (thumbs up for Becky's Cafe on the Portland waterfront) and dinner afterward, and lunches for all shooting days were catered.
The third shooting day started with driving an hour to the former Brunswick Naval Air Station, where we took over a big building to shoot the “virtual hangar” part of the show. This setup lasted the entire day because it was unusually elaborate for a documentary.
Readers probably know about blue-screens or green-screens (aka Chroma-Key), in which TV meteorologists or actors perform in front of a solid color background. It's easy for electronics to remove the background color, so that other images can be inserted behind the talent. The movie Sky Captain and the World of Tomorrow was shot entirely in front of a screen.
But it's no longer necessary to use a green-screen to place people into a digital world, though the alternatives are harder to use. One of those is called difference keying, and it's how Lone Wolf put us in a virtual hangar.
1) During each shot, the Red camera began the take by shooting the hangar without us in the frame, and then we walked into the frame.
That way, each shot held a record of what the background looked like, before people arrived and started walking around. We also ended each shot by walking out of the frame.
2) In post-production, computer processing pulled away the hangar background imagery and replaced it with a transparent layer, which left Ken, Parks and me walking around in a void -- like villains in Superman's Phantom Zone. Computers can cut away the background, frame by frame, because we humans were moving around, but the hangar wasn't. That created a difference between us and the background, sufficient for difference-keying. No color difference is necessary, though it doesn't hurt. Here's a video demonstration of difference keying.
3) What to use for the new background? Drawing on imagery of the real hangar, the special effects company in New York built a 3-D model of the interior space, which closely resembled the real hangar (as in, it had pipes, a floor, doors, lights, and a ceiling with girders) but was somewhat bigger so as to fit the Titanic.
4) Our images were composited into the computer-generated hangar, along with the ghostly Titanic.
Why go to all this trouble? To put real humans and a digital ship in the same three-dimensional space. For example, when Ken waved his arms, the special effects guys in post-production could make the hulk spin, shrink, or rise off the floor. When Ken and I were walking around with a flashlight near the hangar door, all we could see was a concrete floor. Digital magic inserted the rusty boilers later.
One more footnote for special effects buffs who have seen the History show. Normally difference-keying requires a camera to be locked down (meaning, the camera doesn't move at all during the take), but Lone Wolf gave the shots a little fluidity, this way: Whenever the camera moved during a take, it was looking down at us from a height, which silhouetted us against a uniformly gray floor. Such a background is easy to correct, so it was possible to give a little motion to the shot without making difference-keying impossible.