In the movie, set for release on April 22, the two play lovers Marlena, a circus performer and the wife of its cruel head animal trainer, and Jacob, a veterinary student.
"Well, it's a boy thing, right?" Witherspoon, 35, told Entertainment Weekly magazine. "To have dirty fingernails and dirty hair, and his clothes were dirty all the time. It was a nice escape for him to be tan and in the sun all the time instead of the vampire gear."
In the "Water For Elephants" trailer, Pattinson's character appears to get into some messy fun with his fellow circus workers and befriends a real elephant.
"So this big female started sniffing my foot - big female elephant, that is," Pattinson told Details magazine in early 2010. "She sniffed it so hard, it came up off the pavement like her trunk was a vacuum cleaner."
"Then she took my entire body in her mouth," he added. "I was holding on to her head, and as I slowly let go, she tightened her grip really carefully until I'm just upside down in her mouth and she's going through my pockets with her trunk, looking for peppermints. It was the best day of my life."
Witherspoon, known for movies such as "Legally Blonde" and "Walk the Line" had last year praised Pattinson as being "extraordinarily attractive." The two are set to be presenters at the 2011 Academy of Country Music Awards on April 3 to promote "Water For Elephants."
Witherspoon recent married her longtime boyfriend Jim Toth, a 40-year-old Hollywood agent. Wedding guests included Renee Zellweger, Sean Penn, Scarlett Johansson, Tobey Maguire, Robert Downey Jr. and Alyssa Milano.
Check out the trailer for "Water For Elephants" below.