This is a tough question, I'll be short in my opinion. In this day and age, nothing is sacred. They'll use it without your knowledge or permission, because they have the coffers. or not.

In [the] any Art world, I thought it was good enough to simply register the copywrited original [script, art, music...] in mail that's registered mail to yourself — all dated, stamped by the Post Office [government, in Canada, you know you're safe, RIGHT¿] just waiting for the challenge [opening the original - only in a court of law though]./ For Canada, I believe.

However, it'll cost you to take it there.
So, no, hold some cards waaaay back. Don't show much, but have some apéretifs and discuss. Otherwise, it's gone.

I'd ask an Arts specific lawyer or look up an Association, which must have that information, Join. Get schooled from the source.
Best deal.

I thought it was good enough to simply register the copywrited original [script, art, music...] in mail that's registered mail to yourself

This is called the "poor man's copyright," and is not honored or recommended by American authorities, at least. A casual search reveals no information particular to Canada, but I would guess it's no different there.

AH! All the more reason for m to rip open all those registered letters I sent myself while at college.
I can't remember which ideas I thought were so fantastic I needed to have "proof" of time on them. :) Should be fun to look through.