Christain Bale, in addition to being an asshole, is a vegetarian. He is also very obsessive in his training.
He dropped a huge amount of weight for "The Mechanic" by eating apples & drinking coffee. To build up for "Batman" (he did it in 6 months, if memory serves), he worked out like a madman and ate high-protein & high calories every 3 hours. Depending on who you believe, he was taking in from 2500 to 5000+ calories a day.
Steroids are not required for the kind of body he produced. I watched a friend - following a long illness & hospitalization - put on about a pound of lean muscle per day for over a month, going from 130 pounds back up to 185 in 6-8 weeks. All he did was eat, lift weights, and sleep.
By contrast, Bale went from 130 to 210 in about 4 months, then had to trim down to 190-ish. Amazing, yes. But very do-able if you're motivated and have the time, gear, & know-how.
HEAVY resistance work, proper diet, and rest. Bulking is easier for some than others, but anyone can do it.