It’s best for those backstage to be qualified with any number of things but the largest qualification necessary is to be a leader no matter the skill
level in any one or any in general field. If in lighting rehearsal you have a note for the director by way of what you see they don’t even in acting, that’s making the production into art. (Given tactfully presented and received of course plus presented
thru the chain of command structure.)
The
stage manager often is the replacement and assistant #1 for the director. That’s first qualification often and necessary less as a secretary and more often in taking over the show and the director’s intent after it’s up and running and in doing so continuing the artful intent of the director - that's all aspects of the production and should it be as often that the acting side needs more overall supervision, often the
stage manager just might be more useful to be from the actor side in a step before professional production work where the
stage manager is a professional at all types of things in leadership.
Not an easy job this
stage management - hopefully some slack can be given in even not qualified for other areas, they are still kept in the loop, some patience is given them and lots of latitude for still directing you even if not really trained for your area. Their job is still that of the production no matter that of secretary before production primarily and experienced or not. For production they not just run the actors, they run the crew and the production and even if not all knowing, need to become such quickly or at very least kept at the head of the table in while even if experenced about technical matters, they still don’t get it, are still the ones in charged of the production your efforts serve. As the boss for many they need to know what they are doing in all ways not in tech stuff or qualificatitations but by way of leadership and compartmentalization. That's the real key to the show, not technical expertise. Often the more one knows, the less they are able to seperate this from the all absorbing skill in a field they have learned.
The
stage manager in being master of all fields (real master in directing all) no other tech person could even hope for is not realistic. Even a TD is or cannot be a true master of his or her field, they are also a
stage manager leader type also in doing their best but also needing others to help in at least some details. Note of course that the TD is the one technically most experienced in tech just as the director is the most experienced in directing the talent for the production or artistic direction of it. That's different but also exclusive in a way also making one in a field not qualified to direct that of the other. TD's often would probably make poor directors. Head of departments or technical skill also often are best supervising that field than many others at the same time. Gets into generalist or specilist type of things.
The
stage manger proper is not persay on a expert
level on any
level but hopefully sufficient to understand and direct those expected to do the details to get that work done in all areas of their supervision. This or at least in understanding a concept or problem sufficiently and come up with the best solution for solving problems with. When not still, again, it falls to them as still the boss and the task of those working for the
stage manager (qualified or not by way of your respect) still that person in charged. With patience, you can change even the most actor of
stage manager into a tech person due to their own necessity for mastering their own field - not mastering your field, but the basics for understand all fields. First the chain of command with patience for them, than following their directives given expiation of your concerns. Only time not to follow directives is when it’s very un-safe and at that time you first present the problem, re-express your concerns, than only if still not satisfied take it above their head. Specifically not before.
All tech people in a production work for the
stage manager who not only has your little department but the entire show to consolidate into a production. Help those even if not qualified to be doing their jobs. You are either part of the solution or part of the problem. Which would you rather be and would best serve the show you both are working on?
In stating this pedigree from the past I note some reserve on your part for support of this person that not just needs to be the deciding factor in all that goes on but also the support of all departments in all problems and help from those in areas not so well versed in without anything other than support of those reporting to the
stage manager. How versed are you at costumes? Should the head of the costume dpt. also wish for the
stage manager job, what sets your rights to it above their's in if given you don't really sew too well possibly you are not really qualified to
stage manage either. It's an example but if there is a problem with a washing machine, this could cause production problems. I'm sure you would listen to those who use the things by way of advice. What difference is there between your listening to advice perhaps on this subject than that of someone else needing to listen to advice on your own subject of expertise? This much less in dealing with the actors where this
stage manager might have at least a
leg up on it over you and this is half the battle at least. This especially given a trained crew the
stage manager does not have to worry about.