Within the hour

Timing is most of it. A good thought said the moment after the thing is a different act from the same words said next week, and I'd argue the difference is bigger than the difference between saying it well and saying it badly.

In the room. If you're there when it happens, say it before you leave the room. Your partner puts the phone down, and you say it from the kitchen doorway. The meeting breaks up, and you say it to the colleague while everyone's gathering their things. The window for the said version is short because the detail is fresh, they know what you mean without explanation, and there's no build-up. Ten minutes later you'd have to go and find them, and going to find someone makes it an event. Within the hour is the outer edge. After that it's still worth saying, but it starts to need a preamble, and preambles are where the squirm lives.

The same day. If the moment's gone, use the evening. A message is fine. "Meant to say, those slides were the clearest we've had all year, first time I've understood what we're being asked." A message has one advantage over the room, which is that there's no pause to sit through, and one disadvantage, which is that you'll be tempted to redraft it. Don't. Chapter 5 told you why. Write the detail and the effect line and send it.

Not the card. Chapter 11 covered why the occasion is a bad channel. I'd add that the card is where good thoughts go to be generic. "Thanks for everything" and "you're the best" are what people write when the details have gone stale, and they're read as what people write. If you want to put something in a card, put a detail in it. But the card is a supplement, not the place.

The morning after. If a whole day has gone by, you can still say it, and here's the trick. Don't apologise for the delay. "Been thinking about that call yesterday" is a sentence, and it's true, and it makes the delay into evidence that the thing stuck with you. That's a compliment on its own.

Why the hour, if I had to give one reason. Because the squirm grows with time. In the moment, the good thought is just a thing you saw and said. An hour later it's a thing you decided to say, and a day later it's a thing you've been meaning to say, and each of those is heavier than the last. The fastest version is the lightest, for you and for them.