Second round interviews are typically with higher level professionals, so the questions are more focused (and usually more difficult). Don't get flustered or intimidated. Use these tips to nail the callback:

• Find and leverage an ally. At this point, you've probably met someone who can give you the inside scoop -- it could be an HR contact, a staffing recruiter or even your first round interviewer. Find out what factors are most important to your second round interviewer. Politely ask for feedback and pointers to give you an advantage in the next round.
• Stick to your guns. Your interview strategy landed you the second interview -- so you're doing something right! Be confident. Keep your responses and demeanor consistent with the first round.