The actress, 27, arrived in the south of France Thursday in time to dazzle at the red carpet premiere of her film Wendy and Lucy.
"She was very, very excited – it’s her first trip to Cannes," the film's director, Kelly Reichardt, told PEOPLE, adding that they celebrated Williams's festival debut with some "celebratory cocktails.”
On Friday, the actress began promoting Synecdoche, New York, a Charlie Kaufman film costarring Philip Seymour Hoffman and Catherine Keener.
"It actually does feel kind of special to have two films at the same time," Williams said. "The film [Martin Scorsese's Shutter Island, which she's currently filming in Boston] was sweet enough to let me come here and enjoy it."
The actress (who is in town for just 48 hours, traveling without daughter Matilda, 2) added: "[It's] something you can look back on at the end of your life and think, 'That was a cool thing I did when I was 27.' "
Williams walked into the Palais du Festivals laughing and chatting with Keener, their arms linked around each other's waists.