The actress shared a couple pics early Wednesday morning to show off the character transformation ... saying paparazzi photos -- taken last week -- already spoiled the surprise for everyone, so she's got no issue sharing the pics from set.

SS is outfitted in a pair of gray sweatpants and a red tank top ... showing off her powerful arms while making a muscle for the camera.

She's dyed her hair a dark brown, and it's a little curlier than normal too -- nowhere near the straight blonde 'do she's usually rockin'.

Sweeney adds in her caption she's excited to bring the real-life story of Christy Martin to life ... lauding the pro boxer for fighting serious battles both in and out of the ring.

Sydney told Deadline she was stoked to take on the new challenge ... saying she grappled and kickboxed for about a decade -- and, she looked forward to getting back in the ring and training once again.

Of course, Sydney was offered training from the woman she's playing ... with Christy Martin saying she'd show her the finer points of boxing in preparation for the flick. Unclear if she took the former welterweight champ up on the offer.

Sydney's joined by Ben Foster, Merritt Wever, Jess Gabor, Katy O’Brian, Ethan Embry, and Chad L. Coleman.