Competency: Why law? Why X firm? What do you think about X commercial issue? Basically testing your knowledge and competency of the legal profession and why you want to enter it.
Behavioural: How would you react in X situation? How can you draw on your experiences to help you with X? These will be more focused on you as an individual and what skills you have, as well as how you would act in certain circumstances.
Hope this helps!! This may not be 100% accurate but it is how I understand and differentiate between the two.
Hmm oops I may seem to have gotten this mixed up! Daniel's link really helps though so I'd check that out!!
(ps - why is this so complicated haha)