It's not easy being mean.

Just ask Ryan Phillippe, who played the loutish jock in "I Know What You Did Last Summer" and the hipper-than-thou club-hopper in "Playing By Heart."

He barely could stomach his latest role in "Cruel Intentions," a teen update of "Dangerous Liaisons" which opened Friday in area theaters. In the movie, the 24-year-old plays the ultimate snake in the grass, a rich high-schooler named Sebastian who delights in toying with other people's emotions.

At the beginning of the movie, he accepts a wager from his stepsister (Sarah Michelle Gellar) to deflower a chaste transfer student, played by Phillippe's real-life girlfriend Reese Witherspoon ("Pleasantville"). If he loses the bet, Sebastian's stepsister gets his prize Jaguar. If he wins, he gets to bed his stepsister.

"Reese and I had a fight scene where we had to say horrible things to each other for four straight hours," explains Phillippe. "After it was all over, I went outside and literally threw up. It was so emotionally punishing for me."

It was physical punishing for Phillippe as well.

"At one point I was improvising off-camera for Reese," remembers the actor, who shares a Hollywood home with Witherspoon. "I guess I said some pretty mean things, so she came over and slapped me. (Director) Roger (Kumble) loved it so much he incorporated it into the scene. So basically I got slapped around for a couple of hours."

As a performer, Phillippe is good at being bad. But off screen, he's painfully polite. Unlike most actors, he's more than willing to offer up details of his personal life.

"I haven't had a lot of relationships in my life and this is definitely the first time I've been in love," he says sweetly as he sips bottled water in his New York hotel suite. "Reese and I are planning to get married in the near future. We just haven't set a date yet."

This week, the couple announced that Witherspoon is pregnant and they're planning a summer wedding.

"Cruel Intentions," part of a wave of mean-teen movies that includes "Jawbreaker" and the upcoming "American Pie," is the fourth version of the 19th-century novel about sexual gamesmanship to reach the screen.

In 1960, Roger Vadim shot "Les Liaisons Dangereuses" with Jeanne Moreau, Gerard Philippe and Jeanne Valerie. Stephen Frears' "Dangerous Liaisons" (1988) starring Glenn Close, John Malkovich and Michelle Pfeiffer is perhaps the best-known version. In 1989, Milos Forman directed Colin Firth, Annette Bening and Meg Tilly in "Valmont" (1989).

"I watched all three of the previous films," notes Phillippe. "The Vadim one was very campy, which was valuable because there is definitely some campy elements to our film too. We had to push the humor because the morality issues are not the same today as they were in the 19th century.

"There's a lot in our movie that's not meant to be taken seriously. These are super-rich, super-privileged kids, so it's all a bit over the top. ... I thought of 'Cruel Intentions' as being 'High Society' meets 'Kids.' "

Phillippe was on the set of "I Know What You Did Last Summer" when he first received the "Cruel Intentions" screenplay. He immediately passed it along to Witherspoon, whom he'd met a few weeks earlier at her 21st birthday bash. "I crashed her party," he recalls with a laugh. "I went with a friend because I knew there would be free food and drinks. I was just sitting back in the corner pounding away the drinks when somebody walked over and introduced Reese to me."

The day after the party, Phillippe went off to shoot the slasher film. But he and Witherspoon kept in touch.

Witherspoon says that she was attracted to Phillippe because "he seemed so sincere and so not into himself. Too many young actors are so obsessed with themselves, it makes a relationship near impossible."

Phillippe adds that Witherspoon is "incredibly strong and fiercely independent. I find that very sexy in a woman."

Was it love at first sight for Phillippe?

"It was a pretty intense, emotional connection that happened between Reese and me," he says. "There's a definite balance there. Reese is a happy, light-hearted person, but she's also incredibly intelligent. I tend to be considerably darker. She keeps me from getting too dark and depressed and that's essential in a business that's based on uncertainty and insecurity."

Phillippe hasn't known much insecurity and uncertainly lately. Ever since "I Know What You Did Last Summer," he's been accumulating fans and landing prestigious movie projects.

He played the starring role of a wide-eyed bartender in last year's "54" and shared the screen with Sean Connery in January's "Playing By Heart." Next, he'll join Sigourney Weaver and Woody Allen in the comedy "Company Men."

Phillippe hasn't become so successful that he isn't occasionally star-struck in the presence of his idols. After he finished making "Playing By Heart," the young actor was thrilled to come home to a complimentary telephone message from Connery.

"I'll never erase it," says Phillippe proudly. "I've already played it for all my friends and for my dad and for all my dad's friends. It's very cool to hear Sean's voice coming out out of your answering machine."

The son of a mother who works for the government agency AmeriCorps and a DuPont-executive father, Phillippe grew up in New Castle, Del. "I have great memories of Philly," he says. "I saw Pink Floyd at the Spectrum. I saw Black Sabbath. I went to Live Aid. My folks still live in New Castle, so I get back there as often as I can."

Phillippe was getting his golden curls snipped at a New Castle barbershop when he was approached by a stranger who urged him to seek a career in show business. Phillippe took the advice and signed with a Philadelphia agency.