I'm less and less sure that the simple explanation "use imperfect when the action is of long duration in the past" is very helpful. A better test, in my opinion, is whether you would use the continuous (-ing) form in English:

J'ai joué de moins en moins -- I played less and less.
Je jouais de moins en moins. -- I was playing less and less.

From this it becomes clear, that the imperfect version is talking about something that's ongoing while something else (either explicitly or implicitly) is happening. It is a descriptive statement whereas the passé composé talks about the action.

Take the sentence "In my childhood I played a lot outdoors." This is talking about a long period of time but you still wouldn't use the continuous form if you just meant it as a simple one-shot  statement. The same holds for French: you would use passé composé. However, if you were describing a kind of scene, setting up a mood, then you'd make use of the imperfect tense (or -ing form in English).

I was playing outside, the sun was shining and the wind was rustling the leaves, when....
Je jouais dehors, le soleil brillait et le vent faisait bruire les feuilles, quand...

This correspondence between continuous form in English and imperfect tense in French covers a lot of cases but not all. In French, there are so-called verbs of state (verbs d'état), which describe a state rather than an action. These are almost always used in the imperfect. Some of these verbs are: être, paraître, rester, sembler etc. One way to identify a potential verb d'état is to replace it be être. If this is possible, you've probably caught one. Verb's d'état don't have either direct or indirect objects. They describe the state of the subject in more detail, and not what the subject is doing.

Here is a page which I found helpful for identifying verbs d'état: https://www.tutos.eu/4999

Thanks Chris - that's very helpful. And thanks for having put so much time and effort into making such a comprehensive reply!

Super helpful, Chris, as always.  Thank you so much!