The British actor told Vogue that Witherspoon's "raucous" attitude proved to be quite a "shock" when they first began working together.
"In terms of public perception, she's thought of as America's Sweetheart," he said. "She kind of is in a lot of ways."
He continued: "I think that she's a lot bawdier than that, a lot more raucous. It did actually shock me to see that. She's tough. You wouldn't want to get into an argument with her at all.
"You can always tell that she will be incredibly nice to anyone who's not an idiot, but it's always very clear that there's a line you really shouldn't cross."
Witherspoon later responded to her fellow actor's comments, by saying: "Oh, yeah. I'm a little junkyard dog."
Pattinson recently revealed that he fancies older actresses like Jane Fonda and Ellen Burstyn more than young starlets.
Water For Elephants opens in US cinemas on April 22 and in the UK on May 4.
> Robert Pattinson 'cried for elephant co-star'
Watch the trailer for Water For Elephants below: