Blake Lively's getting a lot of hate online from fans who think she's a hypocrite ... 'cause resurfaced behind-the-scenes commentary reveals she improvised a scene where she grabbed her costar's groin.

Here's the deal ... in the movie "A Simple Favor," actor Henry Golding plays Blake Lively's husband -- and, in one particularly intimate moment, Lively grabs Golding's crotch.

While the moment looks scripted, Lively revealed on the Blu-Ray commentary that she actually improvised the moment ... explaining she felt it was something the character would do.

Listen to the clip for yourself ... the director -- Paul Feig -- says he loved Blake's addition here, and Blake jokes it's always awkward to meet someone and immediately ask if you can grab them by the nuts.

Feig says they kept asking Blake to grab Henry's intimate area more aggressively ... saying they wanted a very tight squeeze on his business.

Lots of fans online are using this commentary to call Blake a hypocrite ... remember, Blake's suing Justin Baldoni -- at least, in part -- for allegedly adding more intimate scenes that weren't in the script to the movie.

Worth noting ... Golding talked about filming intimate scenes in the movie during a 2018 interview -- explaining everything was shot on a closed set, and he and his scene partner ran through much of the action before shooting, though they left the door open for some spur-of-the-moment acting.

Back in January, Baldoni's legal team released raw footage from "It Ends with Us" where he and Blake were chatting amicably during a take -- footage Baldoni's team says proves everything Baldoni has said.

However, Blake's team fired back claiming the clip showed "Mr. Baldoni repeatedly leaning in toward Ms. Lively, attempting to kiss her, kissing her forehead, rubbing his face and mouth against her neck, flicking her lip with his thumb, caressing her, telling her how good she smells, and talking with her out of character."

Lively was clearly comfortable letting her character dictate her actions on the 'Simple Favor' set ... so, it's certainly interesting context to her legal war with Baldoni.

There is one critical difference, though, which her camp is pointing out.

A rep for Blake Lively tells TMZ ... "Blake collaborated and reached agreement with the director and her co-star before filming the scene. That is the entire point here and that is what Mr. Baldoni did not do. The audio commentary referenced clearly says that she made a creative suggestion, all agreed on it, and it was incorporated through the appropriate filmmaking process."