Director Imtiaz Ali said Kapoor's clean look is part of her girl-next-door image in his movie, which also stars Indian actor Shahid Kapur.
Bollywood actresses are rarely seen without makeup on screen, and are often criticized for wearing heavy greasepaint even when portraying poor villagers, farmers, lawyers or police officers.
But Ali said Kapoor's new movie is going for a more realistic feel.
The film takes place on a train and follows Kapoor and Kapur through a series of adventures after they are left stranded in a remote railway station without luggage or money.
"Have you ever seen people wearing heavy makeup while traveling? In fact, people don't like applying any makeup,'' Ali told DNA newspaper. "Kareena is one of the few heroines who looks good without makeup.''
The movie is a stark contrast to Kapoor's last film, "Don,'' in which she played a seductress and bar dancer trying to help police catch the mobster who killed her partner.
One of India's top actresses, Kapoor is often cast in glamorous roles. She hit stardom in the 2001 family drama "Kabhi Khushi Kabhi Gham,'' or "Some Happiness Some Sadness,'' and is still remembered for her role as a beautiful, bubbly college student.
Her most recent hit was "Omkara,'' a Bollywood version of Shakespeare's "Othello.''