Farrell and co-star John C. Reilly sat down with ET's Deidre Behar on Monday where they opened up about how Farrell put on 45 pounds in just two months.
"I was wondering, 'How is he doing it? How is he doing it so fast?' and then he's like, 'I just eat Haagen-Dazs or ice cream or whatever,'" Reilly recounted. "I was like, 'But how can you eat that much ice cream in a day without getting a freeze headache?' And without missing a beat, he was like, 'Oh, I just warm it up and just drink it.'"
"In the microwave," Farrell added. "I was always a better drinker than an eater so I just applied that rule!"
Farrell, who said that the high-fat diet was only fun "for about two days," said that his health -- especially his sleep schedule -- suffered quite a bit.
Farrell revealed that it wasn't just eating melted 4,000-calorie meals that helped him put on weight -- he also refrained from exercising or really even moving at all.
The 39-year-old star said his sister, who was on set with him during filming, would ask if he'd like to walk around the luxurious estate where the movie was shot, to get some fresh air.
"I'd say, 'No can do,'" Farrell explained, joking that he'd ask his sister for a "piggyback" ride from one room to another when she would return from her walks.
Now, Farrell appears to be back in tip-top shape, and his method for losing all the extra pounds was apparently very straightforward: "I dropped the weight so quickly just by starving and working out," he explained.
In The Lobster, Farrell plays David, a newly-single introvert who, according to the rules of society, must find love within 45 days or he'll be turned into an animal of his choice -- which happens to be the eponymous lobster.
The surreal comedy, which also stars Rachel Weisz and Ben Wishaw, hits theaters Friday.