Practice

If you're not so confident in your conversational skills, practice.
Though there's totally nothing wrong with being quiet in social situations, in an interview you'll want to talk. Talking and elaborating versus giving two-sentence answers will give the employer a feel for your personality and your passion. Outside of your resume and your referrals, talking with you is the best way for them to evaluate you as a possible employee so practice if need be.

You can practice with a friend. You can practice in front of a mirror. You can even record yourself speaking and watch it over to see if you have any unconscious habits. Find the way that works for you and do it often before the interview.