an actor's line that immediately precedes and serves as a reminder for some action or speech?

Yep. You got it.

Yeah, Oris, for example:

Actor 1: How the hell do you know that? (this is the "cue" for actor 2 to say):

Actor 2 Because I seen it happen, eh? (now this serves as a new "cue" for Actor 3, who busts in and says):

Actor 3: You're both nuts!

Thank you both.

BTW, when reading "Franklin D. Roosevelt's New Deal", would native English speakers usually read as "Franklin Roosevelt's New Deal"? That is, D in the name can be omitted?

Sure, the middle initial aint essential at all--except in cases where one might be confused. For example: George W. Bush distinguishes him from another former president named George Bush (i.e., George H.).

In this case (as with many presidents), the intials alone would suffice: "FDR." But in that case, you do need all three.

No, if it were written, I'd read the "D," too.