Depends on the torrent client that you are using and whether that client allows you to prioritize parts of the file pieces.
I have not done this, but heard about it. In some clients, you can specify which part gets a higher priority. If you download a couple (or more depending on the entire file size) parts first, you can open the incomplete video file in VLC Media player, which should play it - might not play it perfect, but should be good enough to get a preview.
I use BitLord, and many a times, I've tried opening a 80% complete file with VLC. Worked most (or I think all) of the times so far.
Opening the file in any media player will not damage/corrupt it, thats for sure. The file is always open for read operation. Moreover, your BitTorrent client must have already acquired a write lock on that file, so no worries. Even if some s/w tries to modify / corrupt, it won't be able to.