The actor recently dished on what went into learning about his character for the Oscar bait-y flick -- which got rave reviews and even a standing O at the Cannes Film Festival the other day -- and he says what he gleaned from the roughnecks of Oklahoma was "eye-opening."

Watch how Matt characterized the experience during a panel Q&A following the screening of his upcoming movie in France. He explains how he and director Tom McCarthy spent a good amount of time down in the South ... where they got the lay of the land, literally, from guys who do the job day in and day out -- getting a sense of their lifestyles, their POVs on life/family and, of course, their politics as well ... which Matt didn't discuss with disdain.

If anything ... he's got the opposite sentiment, saying the people he was around -- not to mention his character, Bill -- own who and what they are ... even on the issue of MAGA.

It's a pretty fascinating take on what many may perhaps write off without a second thought -- the minute you bring up Trump and his supporters ... a lot of folks will tune it all out.

Not Matt, though ... the way he talks about his encounters down there makes it seem like there's some good people around -- regardless of their politics -- and that they're more nuanced than how they're made out to be a lot of the time. To which we say ... obviously.

BTW, this film isn't necessarily a slice-of-Oklahoma-life type of a project -- where Matt immerses himself in method acting or anything. It's actually an action thriller -- a la, "Taken" -- with the oil-rig/middle America stuff as the backdrop. Still ... word is, Matt's turn here is getting tons of buzz, as they're saying he's "unrecognizable" on screen.