I have been a CNS (or jobs with similar titles) for many years ... and I think one of the biggest mistakes staff nurses make when interviewing or giving input the selection of CNS is that they form this picture of "what would I like best in a resource person to help ME do MY job and make MY life easier?" Then they look for that type of person and won't support anyone else.
CNS's tend to pursue that role because they have a strong committment to nursing and a strong interest in their chosen specialty. But that strength of committment and the perserverence it takes to get that MSN usually means that they/we can be a bit strong-willed. To try to squeeze us into tight little boxes with little flexibility often just results in conflict. Most CNS's need a little room to maneuver and flexibility to meet the demands of ALL our "customers" -- and out "customers" include not only the staff nurses, but also the managers, the doctors, the staff development staff, etc. -- and oh yes, the patients.
While it's not unreasonable to want someone for the position who will help you, teach you, be supportive, etc. be careful to not get toooooo self-centered in your judgements. Most CNS's have a wide range in their responsibilities, such as participation in hospital-wide education programs, serving on committees, etc. It's not reasonable to expect them to drop everything to come and help you every time you get busy. I've seen a lot of really great nurses become CNS's and get quickly become bitter and burned out because everyone in their work environment wants them to be the "perfect helper" for them and them alone. "Ignore all those other job demands. Help ME!"
In other words, don't expect the CNS to solve all your problems for you.
I would ask questions that help you get to know what the candidate values, what type of work he/she most enjoys, and what special professional interests he/she has. For example, what do they value in a nurse? How would they describe "a great nurse?" How would they describe "a bad nurse?" How would they describe a "great CNS?" Are his/her values similar to yours? If not, you're probably going to have some conflicts along the way.
Are his/her special interests and favorite types of projects the types of things that are compatable with your unit? If so, there's probably going to be a good fit between the candidate and the unit. Does he/she bring any new ideas or interesting background that will enrich your unit? The CNS should enhance your unit with new programs, new ideas, etc. not just continue things the way they have always been done. If you just want things to always stay the same, there is no reason to hire someone to just do routine maintenance of the status quo.
Those are just a few thoughts off the top of my head. I'll be interested to read what others think.