Formal or informal? I believe it’s a big question and must be answered for every story. I mean formal is good for a medieval-like fantasy story. But not for a YA-book which is set in this time. So, what are you thinking? #formal or # informal
Formal or informal has nothing to do with the period in which a novel is set or if it's fantasy or modern.
As you suggest- It depends upon how the individual author wishes to approach his telling of any particular story.