Is there a word for speaking of something made-up as if it were real? Like a character from a novel or movie?
This could be speaking in character
26. in character
b. in accordance with the role or personality assumed in a performance
Speaking "in character" works when it's an actor speaking, or a role-playing gamer speaking in game terms.
The more general term you want is "in universe". A common example would be any Star-Trek fan discussing issues as if Star Trek were real. This would include discussions of starship combat tactics, analysis of warp-physics, aspects of various alien races, or all of the different starship designs.
They were having an in-universe physics debate.