Hi,
Someone has asked earlier "what makes or breaks the deal for those interviewed, given the high percentage of candidates interviewed?
The views below are my own personal views. However, I do hope that they shed light on the matter for people who have applied for round 2.
I felt that CMU cares about 2 points: 1) commitment to attend to CMU if admitted, and 2) likability.
1) CMU knows that it is a borderline top 15 school. Thus, the school's yield is lower that it would like to see. Also, Pittsburgh is - ahem - not one of the most glamorous locations. The winter there is harsh, and the city is relatively unknown.
You should give very specific reasons WHY CMU stands out as the best program for you. People with engineering background will have an easier time to explain their fit, but remember: as many as 50 % of admitted students come from an engineering background. You should buttress your preference beyond your background, to distinguish yourself.
2) Likability.
Given the high percentage of the engineering candidates, the most prevalent "personality flaw" is that candidates lack smooth, well rounded social skills. I am not saying this is correct - I am just pointing out a generic (mis)conception that is prevalent.
You would like to show that you are an active, engaging, and likable person to overcome this generic stereotype, especially if you are from engineering background.
To give a concrete example, let's assume that you have won the Nobel prize in Physics. You think that this is a very impressive achievement, that should lead you to be accepted almost automatically. Not so fast.
If you tell the adcom that "well, my biggest achievement is the Nobel Prize in Physics" and remain mum for the rest of the interview, the adcom may not be as impressed as you thought they would. Why? because this kind of behavior just reinforces the stereotype that engineers (or scientists) are brilliant but not social.
Instead, the adcom will be more impressed if you tell them a story that you are a keen mountain climber and that you led a group of teenagers to climb the Everest last summer.
Then, at the end of the story, you add "while seeing the sunrise, I realized the major flaw in the current quantum physics. After going back home, I wrote the thesis that lead me to win the Nobel prise in physics!"
Admittedly, the above example is somewhat extreme. However, I do hope that it delivers the messages I had in mind, that I believe are important to succeed in CMU interview.
Good luck to all!