The Irish actor shed 44 pounds (19.9 kilograms) for the movie and admits his decision wasn't "healthy" or "advisable".
And he needed the caffeine just to get the energy boost he needed to act.
He says, "It was kind of depressing... For a short space of time, it was grand. It was just calorie counting wasn't a particularly healthy way of doing it."
And Farrell insists it wasn't hard to put the weight back on: "I just ate like a pig.".
I prefer my bb with some ~meat~ on his bones!