90% of the time it's gear.
When I started really lifting, I wanted a role model for inspiration. I picked a movie star who recently went through a "transformation" for a role. We were similar height, similar starting weight and physic, so this gave me a good baseline to track against and a realistic goal, I thought.
I read all the fitness magazine articles about his workout and diet. I read the book his personal trainer wrote. I did the same exercises, ate the same food, everything. And my results progressed, and they were similar and getting closer, but they just weren't there.
I wasn't developing certain muscles the way he was, despite doing the same exercises (read flag 1, he was growing muscles he wasn't working). I threw in auxiliary exercises and started to catch up on those muscles. He said in an interview that he got in such good shape, he was able to bench his own bodyweight (170 lbs). I was benching 270 and didn't look as muscular (red flag 2, I was over 50% stronger than him, he was gaining muscle, not strength). He was also filming in Eastern Europe, which I later learned is where steroids are legally prescribed by doctors at supra-physiological levels. Stars apparently go to Germany to get put on good, legal, medically monitored gear.
Edward Norton admitted to gear for American History X. Tom Hardy said "how do you think?" when asked how he got big for Bane. The Jersey Shore guys in the off season are flabby and then joked about "let's go get in shape" and then two weeks later are huge. Stalone was caught by Australian customs with test vials.
Someone like Brad Pitt in Fight Club could maybe pull it off without gear because he wasn't really that muscular, or lean, or vascular. He was just fit. Ed Norton in Fight Club had already done gear to build muscle, so he might be able to just cut down and still look like that.
I read an article from a Hollywood personal trainer admitting that most of the industry is on gear, it's an open secret, but of course not his guys. No, he was the only honest trainer in Hollywood.
If your career was on the line, millions of dollars in movie contracts, wouldn't you do it? Athletes risk getting caught to get the edge. They don't drug test actors. Why wouldn't they do it to get an edge?
So, don't base your progress or goals on celebs, their gains are not realistic and will only frustrate you.