Be wary; there are some spoilers ahead ... near the end of the Amazon Prime flick, Barry's character Oliver bares it all while busting out a celebratory dance -- with fans shocked to learn he wasn't wearing a prosthetic penis.

And, Barry has confirmed no extra props were required ... saying he embraced his "ownership" and "full confidence" -- though only after 11 takes because he wasn't comfortable at first.

It wasn't long before he felt empowered ... asking the crew to re-take the shot over and over again ... adding to Entertainment Weekly he quickly forgot he was in the buff because of the comfortable on-set environment.

Barry also says that disrobing was what his character required ... cause he could do whatever he wanted in the grand manor that features heavily in the movie ... even if that meant stripping to his barest and waltzing around like he owned the place.

"Saltburn" is centered on Oxford scholarship recipient Oliver Quick, taken in for the summer by his wealthy pal friend Felix Catton ... played by Jacob Elordi.