At my last job I held hundreds of interviews and it was a mixed bag, discussions after depended on who my co-interviewer was.
The only time we were really annoyed and expressed it after was when we’d rearranged a date to see a candidate, I’d traveled an hour to see him and he told us in the first 5 minutes he was using us to get in with our competitor. When he left we said along the lines of ‘what a twat, wasting our time.’ We know people do it, but why tell us?!
On other occasions, if someone is clearly not employable, we have always agreed on a ‘shall we cross him/her off?’ Straight afterwards. I remember one girl was so flustered she barely answered any questions, we gave her a test and she didn’t answer any of it correctly and then ran out of time on a writing question, having only written one (poorly constructed) line. We did comment on how her CV was almost certainly fabricated as it was for quite a senior role.
One of the people I interviewed regularly with would close his book in the first few minutes if he didn’t like the candidate. It was clear from then but I’d have to continue the interview as if they had a shot, was always awkward afterwards discussing it with him as he used to moan I’d ‘wasted his time’ by continuing for so long.
I’ve never interviewed and said ‘yes’ straight away, however someone else I worked with did this regularly. She would often say when a candidate left the room ‘well the next interviews are pointless, she’s definitely getting the job’ which again made me feel awful for the remaining ones who thought they still had a shot.
Having been the interviewer, I feel more nervous at interviews but I’m very cautious about things I’ve picked up on. Not touching my hair, good eye contact, chewing gum before hand but not during, always accept water and most importantly - if I don’t understand a question, ask them to elaborate. Nothing more embarrassing than someone answering a completely different question to the one you’ve asked, especially when you really want them to get it right!