The actor (or the people assisting with fanmail) might decide to do you the favor of sending your autograph back in one of their envelopes that has the proper Canadian postage. I imagine that if they try to mail it with your sase that has US postage it will be returned to the actor, unless all you put was your own address. If that is the case I'm not sure what would happen. Maybe the Canadian post office will just add the postage for you. Don't freak out, this happens to people often and some of them have gotten their stuff back anyways.

I stupidly without thinking put US stamps on a SAE that was coming from Canada. I should of used Canadian stamps but had already posted the letter before I realised the mistake.... What will happen to my letter? Will it be posted anyway? Will I have to pay when it finally arrives to me? Will the Canadian post office reject the letter?
I am so devastated I did this as it was one of my favourite actors.