It's a difficult issue since there is no indication in legislation of how co-option is to be done.
My personal preference is to put a short list of general questions to the candidates, asking for written responses. Advise the candidates that the answers will be made public (as would happen in an election campaign). No interviews.
That way, the public can see all the information that was before the council when making the decision. It is preferable to simply proceed to voting in the meeting, based on the written submissions. If the council insists on a debate, I'd be inclined to agree to excluding the public in the interests of good relations, but that is arguable.