Don't get me wrong - I'm not suggesting you become a creepy-stalker-type! People don't like that too much either... Simply be ready to catch the unexpected photo: leave the camera turned on and the lens cap off. And always put the lens cap in the same safe place to avoid losing it. I've lost two over the past 20-something years and it's never a happy thing.
Using a zoom lens helps capture a genuinely candid moment. You have the ability to distance yourself from the subject, yet crop in closely on them. Here are a few examples of catching the subjects being themselves. In each case, they were aware I was in the area with a camera, but they didn't realise they had my attention at that particular moment.
Father and son sharing a quiet moment during a hectic, fun-filled day.
Waiting while his family gets ready for the photo shoot.
Explaining something to Dad.
Beautiful, serene pose. But what she's really thinking about is if she'll get some of that cake and ice cream.
Flour bomb fights. No better time for a photographer to have a zoom lens!
Posed photos are important and will always have their place, but I especially enjoy photographing someone not on display - and then showing them how beautiful they are.