05/04/2013 at 09:25 AM EDT
Whether she's telling off a police officer
("You're about to find out who I am!") or her husband
("I'm allowed to say anything I want to say!"), the dashboard footage from Reese Witherspoon
's April 19 arrest shows off a side of the actress that's, well, not exactly her famous good-girl image.
And boy, is she ever sorry. "She really messed up," says a source. "And she knows it."
At the same time, several people who know her tell PEOPLE in this week's issue that the Nashville-bred star has a feisty streak that is rarely glimpsed in public.
Yes, the incident was "out of character," for the 37-year-old Oscar-winning actress and mother of three, says a friend. But it was "not unimaginable."
"Reese is a southern sorority girl to her core," says a source who has known her for many years. Just 23 years old when she married actor Ryan Phillippe
, "she got discovered young, got pregnant and married young and has no real connections to the real world," says the source.
"With her, what happens in public and in private is very different. She does like to drink" – though not usually to excess, insiders say.
Adds the source: "She's actually the real girl next door, not the cliché girl next door."