I have a huge question that keeps nagging at me. You know, in books, how if someone's saying something and they get interrupted? Like:

"No way I wi--"

I'm getting confused on how to make it correct. Is it two dashes? One? Space before and after the dashes? Could someone explain it. Also, is this the same concept like in:

The beuatiful wings--flashing off in every color possible--appeared within thin air, on the back of...

Can anyone explain this stuff?