@StepTurn: But I will never again be "UI", as I know what my responsibilities are for operation of an aircraft
I disagree. If you step into a 737 you don't know what you don't know. Maybe the on board computer needs to be in mode X with setting Y etc.
For all the known stuff (flap selection, speeds, performance, etc) you are indeed CI, but for all the unknown stuff (and therefor for flying the 737 in general) you're UI. Not knowing what you need to know is UI
(assuming you're not type rated on a 737 of course, as we're talking in the private flying part)