Sylvester Stallone drank 25 cups of coffee a day while shooting Rocky III

'I may have looked pretty good on the outside but inside it was [a] very dangerous thing to do'

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."

Rocky III, directed by Stallone himself, was largely positively received by critics at the time; though it especially made a killing at the box office, surpassing the gross of its predecessor.