My take, for what it is worth!

The “surprise” agony! and ecstasy!! indeed

Sure thing Turnbull The suspense, subtlety “making it more effective and dramatic”
Also it gets and keeps the audience engaged especially fanatics like us for 50 odd years! Otherwise what will we debate!

Pete, It seems to me there are times we don't “get a window into what the [Corleones] is thinking” either

Whilst we have our 'novice' theories, they are not quite conclusive until Checkmate! If my memory serves me right, among others -

Godfather
  • why is Carlo being made Michael's right hand man just because Carlo grew up in Nevada?
  • when Michael tells Tom “You're not a wartime Consiglieri Tom things may get rough with the move we're trying” what is the move they are trying?
  • if the move is taking over the Hotel the Corleones bankrolled for Greene, Corleones giving up all their interests in the olive oil business and settling out in Nevada why may it get rough?
  • when Michael told Vito in the Garden scene “I'll handle it. I told you I can handle it, I'll handle it” what was it Michael going to handle? until the Baptism, Greene and Carlo murders
  • when Michael agreed to Tessio brokered meeting with Barzini at Vito's funeral - oh, no! Has Michael forgotten Vito's warning that “whoever comes to Michael with this Barzini meeting he's the traitor” until Cicci tells Tessio “Boss says he'll come in a separate car” screwing up all Tessio's arrangements!
  • Carlo's involvement in Sonny's death until Michael's “You have to answer for Santino, Carlo”
  • Did we think Carlo had gotten away with it because Michael won't make his sister a widow until seeing Clemenza sitting in the back seat