What is the difference between printing on regular paper and 8x10 paper, and why do casting directors prefer 8x10?
8 inch by 10 inch is the standard for headshots and has been the standard since the beginning of casting. Because of this, it's extraordinarily difficult to change the standard and when someone sticks out from the standard, it can be bad thing. Although they say sticking out from the crowd is good, in this case it's bad because your headshot is literally sticking out from the pile.
This is why in a stack of 8x10 resumes, you don't want to be the noticeable one that is larger than the rest. It makes it difficult for the casting director to carry around a nice pile of head shots and resumes.
I can't stress enough the importance of cutting your resume so it fits neatly on the back of an 8x10 print as well. And make sure to staple all four corners. There is nothing more annoying to a casting director than having to search through a pile of Headshots and resumes to find one that got separated because you only stapled it at one corner. Most of the time instead of spending the time searching for it they'll just toss your photo and with it, your chance to land the role.