The final interview occurs after successful completion of the RC process; it is a crucial step.
This final interview is done before a board chaired by the CO of the unit, and usually has three other officers present, one of which will be from an outside unit
They will confirm your interest and ability through a series of question. The questions typically relate to your education, any previous employment, awareness of current events, and in particular why you want to be an Artillery Officer. Consideration is given to the way you articulate your responses. Although somewhat subjective, it is scored, and you will be given the results.
Should the board conclude you're acceptable,l your file be completed by the RC and only then will you be sworn in as an officer candidate.
I have sat on 4 different artillery officer selection boards since Aug 2011, the best piece of advice I can give you is just be truthful about why you're there.