Spotting on a one off basis is pretty hard. You don't know how they train at all and what they're capable of lifting. Spotting in a smith is stupid. I get the "mental assistance" that comes as soon as someone puts a couple of fingers on the bar but seriously, toughen up and just lift!
I got asked to spot once (usually just ignore kunce), guy wanted 5 reps. Struggled on the second so I started to 'assist' (ie two fingers) on the third... Got yelled at to get off the bar, so I did. He didn't get the rep out and never asked for my help so he racked it on the safety.
Apparently I was the bad guy for not spotting correctly... Told him to fuck off and went back to my own thing.
I guess you've got to communicate but from training on my own, on the rare occurrence that I train with someone and they start talking it up, I just lose focus, crack up and the lift is gone. It probably helps some people though?