"She's a fantastic actress," Weisz, referring to Holmes, told PEOPLE at Thursday's NYDG's Rx Haiti benefit in New York. "She will be fabulous."
With the possibility of critics comparing and contrasting their performances, Weisz insists, "It's not a competition. I'm surprised more people haven't played her thus far. It seems strange there hasn't been many versions. But it's great she has the opportunity to play her as well."
The 2006 Oscar winner (for The Constant Gardener), 40, will star in the big-screen drama to be directed by husband Darren Aronofsky that chronicles the days immediately following President John F. Kennedy's 1963 assassination. Meanwhile, Holmes, 31, will star in the History Channel's eight-hour miniseries, The Kennedys, costarring Greg Kinnear and due to premiere next year.
Will Weisz watch Holmes's take on Jackie? "For sure!" she says.
"It's extremely exciting and scary at the same time," Weisz says of playing the icon. "She's American royalty. So there's some fear."
Though Oleg Cassini was Kennedy's official designer during the White House years, Weisz also looks forward to wearing some vintage Chanel and Dior in the film.
"I haven't really got deep into it yet, but of course she's one of the 20th century's greatest and fabulous fashion icons. I'm thrilled."