Robert Pattinson and Reese Witherspoon Talk Their Water for Elephants Forbidden Romance

UPDATE: Added the video that aired on ET

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Reese Witherspoon and Robert Pattinson share a smooch and more in the upcoming 'Water for Elephants,' and the onscreen lovebirds are talking about their forbidden movie romance with ET! They also share a nugget of obscure movie trivia -- they were once cast in a film as mother and son!

In 'Water for Elephants,' Reese plays the married star performer in a circus who catches the attention of a bachelor animal caretaker, played by 'Twilight' star Rob. When ET visited the Los Angeles set during production on the film last summer, Reese rode atop an enormous elephant for a scene involving an elaborate 1930s parade.

Reese gushed about sharing love scenes with Rob, proclaiming, "I am so lucky. He's so amazing and wonderful and sweet and just the nicest guy too, so you can just add that to the soup. I mean, he's just so good looking and he's a really nice person."

Rob said the pair hadn't yet shot their love scenes, but shared, "Reese has only got two days [of work] left [on the film]. So … they must expect me to be very professional about it." He also gave a tidbit about one love scene -- he says it ends with his character getting beaten up!

"We'll probably just laugh the whole time, because all we do is just laugh and joke around," Reese predicted of filming the scenes.

Although Reese and Rob are an onscreen couple in 'Water for Elephants,' they had a very different movie relationship when they filmed 2004's 'Vanity Fair' -- mother and son! Despite Reese being only ten years older then Rob, he was cast as her son, though his scenes later landed on the cutting room floor.

Reese recalls of meeting Rob back then, "He was walking down this corridor and I turned to the director, and I said, 'That's my son? He's so good looking!' I was like, 'He's so hot!'"

For his part, Rob remembers, "Even then, I realized on the set, I was like, I look too old to be playing her son. I mean, that's probably why I was cut."

Based on the best-selling novel by Sara Gruen, 'Water for Elephants' follows an elderly man (Hal Holbrook) reminiscing about his days working as an animal caretaker at a circus. A young veterinary school student (Robert) witnessing the beauty and brutality of life under the big top, he falls for Marlena (Reese), the captivating star performer, and suffers the wrath of her charismatic but dangerous husband, August ('Inglourious Basterds' Oscar winner Christoph Waltz).

'Water for Elephants' will debut in theaters on April 15, 2011.

ET

First video: Source via @trstno01