I participate in the hiring process at work through phone and onsite interviews. The phone interviews are initial screenings for candidates intended to determine who we want to bring in for the onsite interview gauntlet. Our process has me doing Q&A with the candidate then I write up details of the interview and my recommendations. Then, one or more people higher up the chain make decisions about how to proceed.
For both types of interviews there's a fixed time slot. Sometimes during a phone interview I'm confident within the first 10-20 minutes that the candidate isn't qualified. Often in these situations I like the candidate and would happily hang out with them over drinks and geekery but I wouldn't want my project to depend on them.
I come to the conclusion that the candidate won't fit usually after I've given them a few extra meters of rope to climb up with and they just hang themselves more. If it seems unlikely that more rope is going to rescue them my standard bailout is "Those were all the questions I had for today. Do you have any questions for me?" It's polite but dodgy and it seems unlikely to me that the candidate doesn't realize that this is a signal that they've bombed.
On at least one occasion the "questions for me" portion developed into a friendly conversation about skill development and I was able to provide a book recommendation that flowed naturally with that conversation. What I'd really like to do is just cut the interview, be forthcoming, say "the higher-ups may disagree but I think you still need to develop for a position like this", and then provide guidance about what the candidate can learn and practice to up their game.
I see a few potential risks here. First, it's sufficiently out of normal (impersonal) interview protocol as to feel vaguely unprofessional. That said, is it really important to stay 100% professional (impersonal) when I can actually help someone? It's not just me though, I'm also representing my company.
Second, I'm essentially cutting out the higher ups here and decisions about a candidate's progress through the hiring pipeline are under their purview. But, they make those decisions based on my view of the candidate so if I'm confident the candidate isn't a good fit it would be exceedingly unlikely for them to disregard that.
Third, there's the risk that I might come across as providing a prescription for getting the job in the future. I can mitigate that to some extent by being as forthcoming about my intent as possible. Still, there will always be a candidate who gets the wrong idea despite my best efforts. If I've done my best I guess I can just let it be "their problem" but that feels kind of irresponsible.
The easiest thing to do is just get out of the call and forget about them and that's mostly what I've been doing so far. I was asked recently by a candidate if I share my knowledge publicly and it surprised me to say "no". I used to but I haven't been recently and I feel that's kind of a loss. I should be teaching more.