Okay, I know why pasta absorbs the sauce and it can be dry if left in the sauce, but what about making it not so dry in a casserole you cook in the oven? Did a great casesrolle last night and it was REALLY dry! Do I need to begin with more sauce?
First, you might want to try parboiling (cooking about halfway) the pasta to remove some of the starch and help the pasta absorb some water before combining it with the liquid in your casserole.
Second, if you didn't already, I would rinse the pasta before cooking it in a casserole. Because it is mostly flour, pasta obviously contains a lot of starch, a lot of which gets transferred to its cooking liquid. If that cooking liquid is your casserole, then the starch will combine with the liquid and thicken everything as if you were adding flour to it.
Third, I would cover the casserole to prevent liquid from evaporating during cooking. If your casserole depends on a browned top, just crank up the temperature for the first few minutes and place your casserole on the top rack close to the burner, then after 5-10 minutes, reduce the temperature to the recipe temp and cover with foil.
And of course, you can always add more liquid! This may not be ideal, so I would try the first three techniques above, and only add liquid as a last resort.
What are you making? I make a casserole which consists of rotini, ground beef, onions, tomatoes, and a few other things. I also don't like a dry casserole. What do I do to combat this? I add beef stock. It's a beef based casserole, I add beef stock. Water will 'dilute' the overall flavor of the dish. But if you use stock - well, that just enhances the flavor. Give it a try.
However, in a nutshell, if you want a casserole to be less dry, you need to add more liquid.