"... give me running commentary on what they were doing ..." There's your problem! You are asking people to simultaneously (1) design a divide-and-conquer alg

"... give me running commentary on what they were doing ..." There's your problem! You are asking people to simultaneously (1) design a divide-and-conquer algorithm, (2) write syntactically-correct code on a whiteboard, an unfamiliar mode of writing for most people, and (3) give a comprehensible oral presentation, which creates a lot of social and emotional pressure for many people. Distraction is the death of reasoned thought. Break the interview into single-purpose phases. (1) Help them write down a clear description with some examples. (2) Have them cook up pseudocode or diagrams of how to solve it, with review/debugging from you. (3) Have them write code. (4) Have them explain why it works. As for being crabby, probably a few of them saw, but could not articulate, that the micromanagement set them up to fail. They should handle it better, but it's understandable. If you want to evaluate their willingness to graciously write code, start the interview with a dead simple FizzBuzz style challenge. E.g., write a function that takes an array of integers as input and copies them to an output array in reverse order. Specifically tell them they can ask questions or ask for help, but need not explain anything, that the code will speak for itself.