It is a must-do for my youngest DS and I - we love it and go several times each trip.
PinkBudgie: We go in the off-season, when DCA hours are generally 10 - 6, so not sure in the busier times, but they usually start at 11 and then have a class every half hour. (If you went after the class just started, that may be why no one was around.) As others have said, you can ask at the podium what characters are being drawn, although when we went in May, the CM was a bit snarky (first time ever) and said she could only tell us for the next show - usually they can tell you for the whole day or ask if you are looking for a certain character to draw, and what time that one is scheduled.
You file in and the room is set up like a big tiered stadium, with the artist on a stage in front. There is a big screen that projects what he is doing. You sit on long benches; each space has a big drawing board already there with a fat pencil attached (like the kind Kindergardeners use.) They pass out large pieces of paper that attach to the board so it doesn't move around while you draw. Before you start drawing, they usually show a trailer from an upcoming movie (we saw Toy Story 3 in May).
And as far as drawing ability - they break it way down, no erasers are allowed, they emphasize these are rough drafts, and I am always amazed at how well everyone does. Yes, most are not perfect, but in all the ones I've seen when we hold them up, you can definitely tell who the character is. The class is only 20 min and some CMs go slowly, others quicker which can be a bit frustrating. If my DS feels he's messed it up too much, he "improvises" on the character, with the results usually quite funny, and that way we end on a positive note.
For the newbies, I think Donald, Daisy, and Piglet are some of the hardest to draw - Pooh and Mickey are not too bad.