Extra work is, as I've said before, very much like jury service. The pay's a pittance, the hours long, and there's lots of waiting, but you get to play a small but vital part in a large, complex, and societally significant enterprise. Also you get to spend time with interesting random strangers and see tantalizing bits of a larger story whose beginning and end you may never know.
Being as how I was sitting around the set for a long time with not much to do but watch and think, and being as how I am a writer, I couldn't help but be reminded of a few writing lessons.
The first one came from the fact that, once we extras were all costumed up, you could tell at a glance exactly what kind of character we were supposed to be. This one was obviously a perp, that one a tough cop, that one a no-nonsense detective, that other one a caring social worker. This is no accident -- the casting directors look over photographs of the available talent, choose people to represent the desired type of background character based on their appearance, then select appropriate costumes and props to thoroughly reinforce that first impression.
The writing lesson here is that not all characters need to be fully rounded. The purpose of extras is to make the scene look realistic (it would be pretty odd if our heroes were all alone in the police station all the time), but apart from that they should not be allowed to take any attention away from the main characters. Making them obvious types means that the viewer can see them, understand who they are, and not spend any additional brain power on them. By contrast, I've seen some beginning writers apply everything they've read about developing believable characters to every character, even the spear carriers. This is distracting and counterproductive. You don't want to be too stereotypical -- not every cop is a beefy Irishman, not every nurse an attractive white woman -- but there's no need to build up a life story and background for every person who appears in every scene.
The second writing lesson is that props are a great way to quickly communicate character. I myself had been a police detective in a previous casting call, but this time -- wearing the same suit and standing on the same set, but with a microphone instead of a gun and badge -- I was a reporter. If you have a character enter the scene wearing a stethoscope or carrying a wrench, you've prepared them for action and communicated their role to the reader in just a few words. This technique can be used with major characters as well, to create an initial impression or communicate their current intent.
The third writing lesson is that nothing is deeper than it has to be. When you see a scene with many people bustling about in the background, what you don't see is that every one of them was carefully positioned and began moving just moments before the beginning of the shot. Often there's a line of extras waiting just out of sight, each one with instructions to step out at a certain interval after the previous one. And the wall they're waiting behind? On the side that isn't facing the camera, it's unpainted plywood. In writing, you don't necessarily need to know everything about your characters and your world -- you just need to know a little more than you're showing, enough to create a believable illusion for your readers. (Of course, as you go on writing about the same characters or world you may find that some of your early handwaves need to be fleshed out. This is one of the things that revision is good for, and one of the reasons later books in a series can be much harder to write.)
None of these writing lessons was new to me, and every one of them can be overdone or used inappropriately, but they're all useful techniques and it's good to be reminded of them every once in a while.