In my Sanskrit dictionary, the Latin phrase metri causa ("for the sake of the metre") is used to alert the reader to forms which may be used irregularly in order to fit the metre. For example, in the available literature, a verb might occasionally be conjugated in the wrong voice for metrical reasons, and this irregular usage would be highlighted in the dictionary entry for the root in question.
I was wondering if there exists a formulation that would mean "for the sake of the plot", as when something happens in a novel apparently purely to advance the plot and which feels forced and poorly motivated. If not, can we coin one?
Using some online dictionaries (one, two), the two possibilities for "plot" that I came up with are argūmentum and āctiō, giving us argūmentī causa and āctiōnis causa as possible translations of this phrase (if my declension skills are to be trusted (by which I of course mean, if Wiktionary's declension skills are to be trusted.))
The dictionaries suggest that argūmentum is more commonly used for plays and its English daughter 'argument' makes it seem like a risky choice to me. Āctiō also seems to be used more of drama and its closeness to 'act' is tangible, although apparently they are not directly related. It seems like the better choice, though, as the dictionary defines it as inter alia:
the action, the connection or series of events, the plot
which seems quite suitable.
However, my two years of Latin were ever such a long time ago, so I'd welcome the input of somebody more familiar with the actual language, in order to get something that seems natural. I'd be especially interested if anybody can point to an actual formula that might have been used, either by the Romans themselves, or later European writers.
(P.S.: This is more for my own amusement, I wouldn't use contrived Latin phrases in a real essay, I'd stick to plain English or bona fide Latin phrases (see what I did there?))