I was fortunate enough to speak with Luke Goss while he was in town for the HorrorHoundWeekend, and we talked about, among other things, working with Guillermo. I have never seen GDT on the set, but I told Luke that I bet he was pretty demanding on his actors and crew. Luke said that was definitely the case, and told me a short story about having to spend an entire day on set sleeping on a bean bag chair while in full costume because Guillermo wouldn't let him leave (just in case he was needed). But he added, "I would do things for Guillermo that I wouldn't do for any other director."
A great director holds his cast and crew to the same high standards that he holds for himself. That can be very difficult at times - but there is an understanding that it is not about being a "difficult boss" - it is about demanding the best effort and the best performance to produce the highest level of achievement. Nothing less than that should be acceptable.
I believe this is a big reason why GDT is considered one of the best in the business right now.
As a side note, if you are not familiar with Luke's work, he is a fantastic actor. I think his performances as "Nomak" (Blade II) and "Prince Nuada" (Hellboy II) are mindblowing. Please take the time to visit his official site at http://www.lukegossforum.com/.