Sounds pretty, doesn't it? French often does, after all. =) And after taking a few years of it, I know upon seeing this phrase that it means something like "living picture." I also knew when I read that these tableaux were often put on in the 1800s that it was some kind of performance, and that they were used as fundraisers in the South during the war.
As for what kind of performance--yeah, I had no clue. So if you do already, you're a step ahead of where I was a couple months ago. ;-)
My first thought was that it was a play. In fact, my assumption was that it was a play, and I started writing it into my story as if it were. My heroine was preparing a script, casting her friends in the roles. Then something went "clang clang clang!" in my brain, and I thought, "Hmmm, I'd better actually look that up."
Good thing I did! Some quick research showed me that a tableau vivant is something unique. The performers would select a well-known painting or other work of art (statuary, etc.) and then mimic it. Strike a pose to imitate it, their costuming reflecting the original work of art, and a narrator would say something about it. Then there would be a change of scenes, and the performers would move into another pose, another painting.
Pretty neat, huh? Of course, it threw a wrench into my plan for my writer-heroine to be putting her brilliant wordsmithing to work . . . so I had to get creative--er, have her get creative. ;-) In my story, my heroine, Cordelia, writes a story to pull together these well-known paintings and has gathered all her friends together (those left in the city, that is--most had already fled inland, away from those blasted Yankees) to strike the poses. Their goal--to raise funds for the Confederacy. It is, of course, a smashing success.
Tableaux are still put on today, so some of you are probably familiar with this interesting type of performance. But I can honestly say none of them had made it to my neck of the woods, so it was fun and interesting to learn about them, and integrate them into my story. Vive les tableaux!