I would say Rewatch Bonus or The Ending Changes Everything.
As a discovery writer, I often don't know my ending until I have written 50% or even 70% of my first draft. So when I am done I actually go back through and look for moments in which I can rewrite a scene for foreshadowing, or add Rewatch Bonuses, sometimes just by modifying dialogue or adding an ...
What came to my mind immediately is
Foreshadowing is the art of giving "hints" of what is about to happen in order to build tension, and a pretty common literary device.
In your example, there may be multiple moments building up to the eventual death of that character.
I'm stretching the definition here, and i'm aware of it. When you show ...
The term for this person is the interlocutor, from the Latin. It means the one who "speaks between," and often used for a character in a dialog --for example, the Platonic dialogs --whose role is secondary to the main speaker.
If the person only asks questions, you could also call them the querent.
A member of a team of comic performers who plays a supporting role by helping to set up jokes and punch lines through engaging in preparatory dialog with the principal comedian.
See also: https://en.wikipedia.org/wiki/Straight_man
I don't believe there is a single term for this kind of character. The terms usually applied to those characters roll in relation to the protagonist are Foil, Confidante, and stooge -- or as I call them Chumley.
The Foil serves to highlight the protagonist's qualities and make them stand out stronger by the comparison.
The Confidante permits deeper ...
I believe in this case, a 'sounding board' fits the bill, simply a person to bounce concepts, dialogue, and ideas off of. Just how some characters act as nought but mouthpieces, this one acts as nothing but an earpiece.