Proactively address the low grades. Waiting for the interviewer to bring up the subject of poor grades can cause anxiety during the interview and put you on the defensive. Raising the issue yourself is empowering and allows you, to some degree, to control the parameters of the discussion. Taking the initiative to address your low grades will impress the interviewer because he will know that you understand their significance, that you are not hiding anything and that you are mature and confident.
Explain the reason for your low GPA. Low grades have various causes ranging from taking an advanced class to the loss of a loved one to focusing on non-academic, but important, activities. Telling the interviewer what caused your poor grades removes the guessing game on his behalf and allows him to decide whether you can be successful in law school. If a low GPA was caused by excessive partying, tell the interviewer this as he will respect your honesty, but reframe the discussion by outlining what you learned from the experience and how you have changed for the better.
Focus the conversation on academic and extracurricular achievements. While it is necessary to address a low GPA, doing so should be a brief exercise. Spend the bulk of the interview demonstrating to the interviewer what you have achieved both in and outside of the classroom, whether it is sports participation or part-time employment, by pointing to your overall record. Ultimately, the interviewer wants to know if you are capable of contributing to the academic and social environment at the law school. Give the interviewer examples of how you have done this during your undergraduate career.