The idea was that you learned a lot about something, then chose the telling details. That was the tip of the iceberg. The rest, the other stuff you learned, was presumed to be just below the surface.
As this foreword to All the Kings Horses says, "Those who learn too much about the past are condemned to repeat it. That is, those who have carefully studied eg. 17th century Flemish butchers as 'background research' for their story are often condemned to tell us every little thing they have learned about butchery, the Flemings, and the 17th century in general. They may flatter themselves that this is precisely what they've avoided. They may assure us that what they show in the narrative is only the tip of the iceberg, and that the vast bulk of their research is actually submerged, unstated, implicit."
With the Internet, my iceberg has melted. Characyer needs flashback to backstory of stay in burn unit? Type in some key words in Google. Spend 15-20 minutes reading about various people's stay in burn units, or articles about burn units. Find a telling detail or two - like burn units smell like minty sour cream from the Silvadene salve they use - and drop them in. And move on.
I still get books about big things - I must have read a dozen memoirs about what it's like to be blind, plus tried and failed to work my way through the really badly written thriller Eighteen Seconds - but for the rest, well, what you see is what you get. The iceberg has melted.