I've always worked on the assumption that, whichever seat I'm in, I trust the other guy/gal to look after me and get me out of the brown stuff into which, sooner or later, I will inevitably insert myself. In return, I will do my best to do the same for him/her.
We both make mistakes because, surprise, surprise, we're both human. There's no shame in having mistakes pointed out. I don't try to score points off the other guy unless we have the sort of relationship that he can tell that I'm teasing "Nyahhh, you did such-and-such - beers on you tonight" and I won't tolerate someone trying to score points off me (or any other member of the crew). That is NOT good CRM.
There are several aspects needed over and above those needed by any pilot for someone to make a good captain.
Simple flying skills are, of course, one of them. Yes, they should be superior to those required by an FO.
Good decision making skills are another.
Further, good team management.
The list, of course, is endless, and I don't propose to divert the thread in that direction. But there are as many ways for a company to decide who gets to go on a Command Course as there are ways to skin a cat. Almost universally, I would hope, those companies who look first to seniority would still pass someone over if his skills are not up to the job, whether before the course or during/after it.