Acting has always been a craft requiring intense concentration and dedication - but maybe not 25 coffee cups worth of laser-focus.

Sylvester Stallone took to Instagram for a little bit of a flashback, revealing some surprising behind-the-scenes facts about the making of 1982's Rocky III, in which the actor had to undergo a "high protein diet" to get the right physique for the role; one that he admits was "a very dangerous thing to do".

The actor posted a photo of himself doing a handstand in the corner of the ring in-between takes so that he could get blood back to his head and continue shooting the film's intense fight scenes; specifically his bouts with Mr. T's formidable, "superstrong" Clubber Lang.

"During the period, I only ate very small portions of oatmeal cookies made with brown rice and up to 25 cups of coffee a day with honey and a couple of scoops of tuna fish," he admitted.

A deeply inadvisable, dangerous lifestyle even just for the film's shoot; as Stallone stressed, "I may have looked pretty good on the outside but inside it was [a] very dangerous thing to do. But I wanted the movie to be about change. How people have to adapt to different challenges because if they don't they will be conquered."