This is probably skewed toward highly competitive states, but based on DS's and other's nom's we know there are a few questions you will want to have sorted as they are very likely to be asked:
1) Why do you want to attend USMA?
2) Why do you want to be an officer in the Army? (Or are you sure you are prepared to serve 5-10 years after graduation, etc)
3) What is you plan B?
#1 is a trick question, as (my read) they are waiting for you to say because it's better than ivy leagues, etc. Hint: It's not about college
But ultimately, this is not about prepping for nom interviews. If you do not know the answers to 1-3 well, and are comfortable with them, then you need to do some soul searching ASAP.
The boards in our area seem to view it as their duty to send the best candidates as measured by who will leave as the best officer to serve their country. IE: Who is best prepared to survive and learn what USMA needs them to, in order to serve as a high performing officer in the Army.
Just being able to do well at USMA is not enough. Nor is excellent leadership skills but without the other aspects you have to have to survive, much less thrive at USMA.
Some will probe on your EC's. Eagle scouts will be asked not what they did on their Eagle project, but what they learned. What was the hardest thing. What would they do different. Same for CAP, Varsity captain, whatever.
Likewise, in competitive states you'll likely face some rank. DS faced a two star general, Full Colonel, two Majors and a Captain on one board. His lighest board had Captains to Lt Colonel, with one Merchant Marine civvy. (USMMA grad, however).
These people are hard to BS, so don't try!
They have seen it before!
One other thing, I've not heard of any in our state that were hostile.
All were friendly and respectful. But they do ask hard questions.