Vrftsjtryk

We do interviews then send selected candidates to panel interviews.

Considering the following scenario:

If an applicant comes into the building after they receive a rejection letter, and asks you in person why they were not selected to continue on in the interviewing process.

What would be a suitable response or course of action if this happens?

First, the person would actually have to get into the building. To do this, they typically need to pass the front desk, and there is no reason for the front desk to let them in. That's neither rude nor inappropriate, but perfectly normal when someone shows up without an appointment for whatever reason (interview related or not). "Sorry, the people you want to see are busy".

If they try to force their way in, call security or the police.

Give them no information. If they press, refer them to the information in the rejection letter or refer them to contact the recruiter they were working with at your company (assuming it wasn't you). If there are signs that make you uncomfortable, do not hesitate to call your building security or the local police or law enforcement.

Giving ANY information is not going to benefit you personally or your employer. Your employer has already covered all the bases they care about in the rejection letter. That process exists for a purpose, you should respect that. Giving the candidate info likely won't benefit the candidate, either - if they're not fit for the job, giving them info on why won't suddenly get them hired. And if they want to "improve for the future" they certainly can, and should, focus on getting employment support elsewhere (ie a third party recruiter, job training, etc), not from an employer who has rejected them. Your obligation to interact with this individual ended when they got the rejection letter.

Job interviews and hiring processes can be stressful on candidates, and can bring out the worst in people who aren't equipped to handle the stress. You don't know their mental state or how they handle negative feedback, and in a situation where they've just randomly shown up and confronted you, they're already showing signs that they may not be the most stable, by-the-book person. Think of your own personal safety first and do what you can to de-escalate, end the discussion, and get outside help from security or police as needed.

Provide feedback if you feel comfortable doing so. If you don't simply say you can't divulge that information or give them some sort of canned answer like "we felt other candidates were better suited for this position" and usher them out the door. If you're worried this candidate might get angry and come back with a machine gun or something then alert the appropriate authorities.

Your organisation should have a process for this. Usually the information they can be given is exactly the same as that they would have received in the letter turning them down, or also quite commonly they will be turned away unless they have an appointment.

In the UK we have a requirement to provide interview notes to the interviewer if they ask, but typically that is all.

I think it goes without saying that this kind of behaviour is unusual and probably inappropriate in most cultures, but I don't think that needs to preclude giving the person the information that they're seeking. Perhaps even more so in this situation as the individual might not be aware that they're breaching social norms - in all likelihood their social blind spots didn't help them during the interview process either.

I don't think there is any harm in having a conversation with them, and using the time to explain your decision and what the can do to improve their chances in future. Who knows, they might impress you down the road and the positive conversation might lead them to succeed at getting a job at your organisation in the future.

This would also be a good time to point out how inappropriate it is for them to show up like this, and that they would have improved their chances by simply calling or sending an email for further clarification.

I think the other answers are needlessly paranoid; there are plenty of completely innocent explanations for this. Disclaimer: I don't live in the US.