Thanks for the response! That is what I thought as well, but I have not found a way to apply the 50% opacity across multiple text boxes at the same time, so I am left with sequential fading that I'd really like to avoid, if possible (I find the gradual fade a little busy, and this is a rather important presentation for me, so the details matter a lot).
Is it possible to apply actions simultaneously? If not, I'll just duplicate the slide however many times & have each subsequent slide set up with the desired fading - I just got the impression that Keynote was designed for such things and I should learn how to do it properly!
With Keynote, there are three options for how the actions and builds are delivered. You can have them happen On Click, Automatically with Previous Build or Automatically after Previous Build. They are all self explanatory and so the one you are looking for is Automatically with Previous Build.
What you need to do is apply the Action Opacity to the text boxes you want faded. Then click the More Options button at the bottom of the inspector. Select the first Opacity action and make sure that it is set to Start Build on Click (this is the default so it should be set this way). Then select the rest of them and change the Start Build to Automatically with first build.