When Castle is freezing household objects, Alexis and Martha are sitting at the table talking. When the camera is facing Martha, she is seen holding an iPod, but when the camera is facing Alexis, Martha is holding a wine glass.
It is true that liquid nitrogen can freeze flesh so solidly that it will shatter. However, it would take a lot more than would have been available in the kitchen to freeze a human body solidly, and would require full immersion for a period of time. (You can stick your finger into LN2 and hold it there for some time without being more than slightly chilled; it boils away very quickly and the vapor insulates your flesh from the cold liquid.)