Sometimes when I visit friends and don't have my Mac around I still would like to watch some of the movies I purchased on iTunes together with them. How do I do this? Can I share movies with others just as I can share Apps?
You can authorize up to five computers to use your iTunes account. If you have not used all five computers and you trust these friends, you could authorize their computer on your account, watch your movie, and then de-authorize the computer. In the time when their computer is authorized on your account, they would be able to make purchases using your account, however, so be careful.
Yes, you can.
Even if your friend's computers are authorized they would still need your password to purchase movies.
If you have used all of your 5 authorized computers you will have to either remove them from each individual computer via iTunes:
Or, log into your account and deauthorize all the computers at once: