Lavigne became interested in appearing on television and in feature films. The decision, she said, was entirely her own. Although her years of experience in making music videos would be to her advantage, Lavigne admitted that it was the singing that actually removed any fear of performing before the camera. She specifically mentioned that the video "Nobody's Home" had the most amount of actual "acting" in it. Her first television appearance was in a 2002 episode of Sabrina, the Teenage Witch, performing "Sk8er Boi" with her band in a nightclub. She later made a cameo appearance in the 2004 film Going the Distance. The main characters bump into her backstage at the MuchMusic Video Awards after her performance of "Losing Grip".
Lavigne at the Los Angeles premiere of Over the Hedge on 30 April 2006.
She moved into feature film acting cautiously, choosing deliberately small roles to begin with. In November 2005, after going through an audition to land the role, Lavigne travelled to New Mexico to film a single scene in the 2007 film, The Flock. She starred as Beatrice Bell, the girlfriend of a crime suspect, appearing alongside Claire Danes and Richard Gere. Gere gave Lavigne acting tips in-between takes. On her role in The Flock, Lavigne said, "I did that just to see how it was and to not jump into [mainstream acting] too fast". The Flock would not be released in American theaters, and because it would not be released in foreign markets until late 2007, it would not be considered Lavigne's debut. The film made $7 million in the foreign box office.
Lavigne's feature film debut was voicing an animated character in the 2006 film Over the Hedge, based on the comic strip of the same name. She voiced the character Heather, a Virginia Opossum. The process of recording the characters' voices was devoid of interaction with other actors. Lavigne stated, "All the actors went in individually, and [director] Tim and [screenwriter Karey] and directors were there with me every time I went in, and they made it go so smoothly; they made me feel comfortable.... That was the interesting part, going in by yourself, with no one else to kind of feed off of." Lavigne found the recording process to be "easy" and "natural", but she kept hitting the microphone as she gestured while acting. "I'd use my hands constantly and, like, hit the microphone stand and make noises, so Tim and Karey had to tell me to hold still.... It's hard to be running or falling down the stairs and have to make those sounds come out of your mouth but keep your body still." Lavigne believed she was hired to perform Heather due to her rock star status. "[The director] thought I'd give my character... a bit of attitude". The film opened on 19 May 2006, making $38 million over its opening weekend. It went on to gross $336 million worldwide.
In December 2005, Lavigne signed on to appear in Fast Food Nation, based on the book Fast Food Nation: The Dark Side of the All-American Meal. The fictionalized adaptation, directed by Richard Linklater, traces fast-food hamburgers contaminated with cow feces back to the slaughterhouses. Lavigne starred in her on-screen acting debut as a high school student intent on freeing the cows. The film opened on 17 November 2006 and remained in theaters for 11 weeks, grossing $2 million worldwide.
Both Over the Hedge and Fast Food Nation opened at the 2006 Cannes Film Festival, which Lavigne attended. Lavigne felt honoured to be able to attend and was proud of her work. When asked if she would pursue her film career, she stated that she wanted to take her time and wait for the "right parts and the right movies." Lavigne was aware of the roles she had chosen. "I wanted to start off small and to learn [that] I wouldn't just want to throw myself into a big part." In August 2006, Canadian Business magazine ranked her as the seventh top Canadian actor in Hollywood in their second-annual ranking Celebrity Power List. The results were determined by comparing salary, Internet hits, TV mentions, and press hits.