Reese's 33rd birthday surprise

MANILA, Philippines - Reese Witherspoon had to work on her 33rd birthday last March 22 — but she got a star-studded surprise all the same.

Reese arrived the Los Angeles premiere of Monsters vs. Aliens — in a shimmering, super-short dress by Rodarte — and had just made her way into the Gibson Amphitheater when she and her co-stars were called to the stage by DreamWorks studio head Jeffrey Katzenberg.

After Seth Rogen, Kiefer Sutherland, Rainn Wilson, Paul Rudd, Hugh Laurie, Will Arnett and Amy Poehler gathered onstage with Reese, Rainn announced that the group was about to stage an “intervention” for the Oscar-winning actress.

“We have some hard facts to run by you, because we know it takes a full bottle of Nyquil for you to fall asleep at night,” Rainn told Reese, “so there’s something very special that we want to say.”

He and his fellow actors then launched into a rendition of Happy Birthday, joined by the theater audience, as a cake was wheeled onto the stage. Amy punched home the final verse, singing, “Happy birthday, Reese Witherspoon, Happy birthday to you!”

“Thank you for coming to my birthday party. I mean, the screening of Monsters Vs. Aliens,” the slightly taken-aback Reese told the crowd.

She told reporters while promoting the film two days earlier that she was looking forward to the birthday. “It is a great thing to be able to be doing things with my kids and having them be proud of me. What’s better than feeling like you’ve accomplished things in life?”

Reese said that her new film in particular has increased her cachet with her kids — daughter Ava, nine, and son Deacon, five. “My kids managed to maintain their coolness for a while, but literally it has hit fever pitch at my house. They are rabid and they wake up in the morning (asking), ‘When can we watch this movie?’”

Monsters Vs. Aliens is now showing in theaters.