Also remember that footage shown in a preview is NOT guaranteed to make it into the final cut of the show. Just because they choose to use it for the preview doesn't mean they have to use it in the final cut. Same thing goes for movies. With all the "raw" footage that is obtained from the initial taping, they choose a few snippets here and there to make the preview. The snippets they choose are chosen specifically to give the viewers a (more or less) accurate idea of what the next show will be like. The week that you see a preview, in all likelihood they're still editing the actual episode in post-production. Lots of stuff needs to be cut out, and, like I previously stated, snippets from the preview won't always end up in the final cut. It's just how editing is done.