Yes, the recordings get done over many times. Sometimes we lose someone due to story changes or some other circumstance. This is a bummer, as there is more than the voice that is lost. While it is easier for some stars to drive over to Glendale from wherever they live in LA, they still "perform" the part and that performance is recorded. The video of the voice over work is referred to by the animators and influences a lot about the how the animated character behaves. I suppose in less expensive productions, you could just try and plop a voice over in, but all the top studios will use the video reference and ask the talent back for voice fixes.

In general, I found the talent to be really fun to be around and no-one just phoned it in. They really liked to work on the films. I ended up sitting next to Gerard Butler at the employee screening of Dragon at the now non-existent Universal Amphitheater. He stayed for the whole film. Unlike me, he hadn't seen it in thousands of tiny chunks during dailies, and had a blast.