The following is from Prison Talk. It is a 2007 thread, but should still give you some good information. The respondent says he is a defense attorney who use to be a prosecutor. ?? Anyway, it's something to go by.

"It is an interview conducted by the Department of Probation, much as a Pre-Sentence Investigation is conducted between plea and sentencing on a felony. For starters, a PPI will only be conducted if a judge allows it. A DA can be opposed or not (I used to hate them, personally, back in those days, as it demonstrated that the judge did not hold my opinion in high regard), but the judge can decide. Consider yourself lucky if you have a judge who will allow this. I have a couple of caveats (warnings) for you, though. 1) As its full name suggests, a "Pre-Pleading Investigation" anticipates a plea. The investigation is undertaken in the hope of humanizing the defendant to the judge. However, the very concept necessarily suggests to the judge that the defendant is ready to accept accountability in the form of a guilty plea. There are judges who will be mighty displeased if after the investigation, a plea is not entered. 2) To cover your bases in the event that the judge will not do as you hope after a PPI, it is best during that interview not to discuss the crime. You can get away with this by simply saying, "My lawyer said it was best not to talk about that at this point." Remember, the right to silence remains until the plea itself."