"If you knew what was best for you, then I reckoned that you better refrain yourself from talking at all, later during the trial. That was of course unless you were asked to speak. You must talk if you were told to. But be extra careful about what you say once we're there since it might be used against you instead. The lesser you talk, the lesser the chance for a mistake to happen. I hope that I had made myself clear."
Jace advised Greg out of goodwill.
As a standalone prosecutor/ death guardian, part of grim reapers' job was to make sure that every deceased soul under their jurisdiction would get the best available kind of afterlife that they deserved.
So to make things easier for him and to his client, Jace took the liberty of giving Greg a useful piece of advice.
In his millennia of service, Jace had already come in terms with all the possible aspects that could help his client to pass the trial.