Tuesday evening, the two actors were spotted flirting with each other at the Chateau Marmont in Hollywood.
According to People magazine, Miller, 25, and Hartnett, 28, spent the evening together before leaving just before 1 in the morning.
During the evening, Hartnett was holding the actress around her waist, before the two disappeared into a pool-side room of the hotel.
A spokesperson for Hartnett, who was recently dating actress Scarlett Johansson, told the magazine, "They’re friends, just friends," Miller’s rep tells PEOPLE. Seconds a rep for Hartnett: "There is nothing to comment on."