I think one of the key traits of a writer is nosiness. Sorry, keen observational skills. Not to mention surreptitious note taking on napkins and receipts. I usually carry a notebook, and I find quick photos with the mobile phone also very helpful.
We were stuck at a service station a few weeks ago, in a Costa, having a tea break. A dapper man in his fifties or so came in with a younger woman, maybe early thirties. they were very affectionate and flirty. He was wearing a wedding ring, she wasn't. I could hardly sit down and ask them, in a non-judgemental, curious way, 'are you having an affair?' But I could shepherd my husband away from the inviting sofas and towards the hard backed chairs right behind them. Her name was Cheryl, I think, but it might have been Cherie for all I know. (Maybe he was French!) She just called him darling, about twice every sentence.
Anyway, being a bit road-dazed, it took the husband a while to realise what I was doing (I think my actually pulling out a notebook may have tipped him off.) But, so much research! He can't complain, he writes songs on the fly, and once wrote a love song that you realise slowly is a stalker talking about his victim. I think he got that off something written in the sand on the beach.
PS This is not an endorsement of Costa coffee or of affairs. I don't drink coffee, for a start.