In "Knives Out", a mystery, it's asserted: "The complexity and the grey lie not in the truth, but what you do with the truth once you have it." What does it even mean?

Example:

The truth: various senators stabbed Julius Caesar. Various other existential observations of the circumstances.

The complexity and the grey: all sorts of ideas, reasons, justifications, lies and their effects, thinking, rumors, circumstances, unclear moral dilemmas, historical accounts, Shakepeare’s re-telling, etc., none of which are understandable only from looking at the truth.

Also, in Knives Out, there are issues from various perspectives about what’s known or thought known to be true, and what makes moral sense to do about it.

I’d say the statement and the film are somewhere between the extremes. Clever, interesting and entertaining in some ways, and also itself a bit superficial and forced/contrived/nonsensical in others, if you think about them closely enough.

Another example:
Nazi Germany. You find out that your neighbour is hiding a Jew in his attic. The Gestapo knocks on your door and asks you, if you have seen anything suspicious. What do you do?

@ragingloli: But the truth can also be complex, right? And grey, too.

It’s very similar to the outcome Poirot was faced with in Murder on the Orient Express.
In both movies the case was solved, but the truth was a complicated matter in terms of justice, as the law sees it & posed a moral dilemma for the detectives in charge.
The truth was that the perps were entirely vindicated in their actions & as such, innocent.

@ucme: My point is that the motive must not be the only thing complex. There’s also the modus operandi.