The actor normally follows a healthy eating plan for his svelte sleuth role in U.K. TV show Sherlock, but he decided to let loose and gobble junk food after he was cast as an English aristocrat in new BBC series Parade's End.
He tells Britain's ShortList magazine, "We had some fun times when we were filming in Belgium. I was desperately trying to put on weight, so there was a lot of (eating) rubbish food and drinking alcohol without worrying about it.
"With Sherlock, it's lots of seeds, juices, swimming and running, but on this I was doing lots of beer, wine, chips and the most f**king amazing proper steaks and goulashes. I still didn't put on enough, though."
Cumberbatch did manage to bulk up for the upcoming Star Trek sequel, but he's adamant weight gain doesn't necessarily enhance an actor's performance.
He adds, "For Star Trek, I went up about three suit sizes... But I've always been a bit po-faced about (the idea) that all you need to do to be put in the hallowed halls of method acting with Marlon Brando and Robert De Niro is put on s**tloads of weight. Come on!... The effort involved deserves some credit, but it doesn't make a performance."