The actress, who announced last month that she'd be returning to her Cruel Intentions character for NBC's reboot, shared a photo from the set Monday.
In the mirror selfie, taken in what appears to be a makeup trailer, Gellar looks chic with her hair pulled back into a chignon and wears a black top and printed coral skirt.
"Don't think she's much of a jeans and tee shirt kind of girl," Gellar, 38 wrote on Instagram, adding the hashtags, "#Crueltv #makeup #kathrynmerteuil."
VIDEO: When Did Reese Witherspoon Lose Her Virginity?
The actress just dyed her blonde locks brunette – diabolical society girl Kathryn's hair color in the original 1999 film.
NBC's series will center on teenager Bash Casey (Taylor John Smith), the son of the late Sebastian Valmont (Ryan Phillippe in the movie) and Annette Hargrove (Reese Witherspoon). Kathryn – Sebastian's stepsister – will attempt to take control of Bash's family's company Valmont International, as well as manipulate her young nephew.
Over the weekend, Gellar also shared a peek at the pilot's script, which was written by the film's original director Roger Kumble, as well as Jordan Ross and Lindsey Rosin.