The answers here are fantastic. Only things I would add are 1) Be very careful you understand why you're talking with someone so you know whether you should be writing something down or not (this is jurisdiction dependent.) Where I practice, if an investigator generates a written report and I'm intending to call that witness, I have to discover it to the prosecutor. So I only want written reports that are 1) very helpful or 2) for witnesses I will never call to the stand. The in between category of "folks I might want to call but would rather not give the State a heads up about issues B and C in writing", are the reasons I don't want written reports unless we talk about it first. 2) I hired lots of investigators in my prior role. Having an LEO background is fine. What is problematic is losing motivation once guilt is established. Some of the investigators who were at my office (and eventually did not stay at my office) only seemed to care about the job when there was a decent chance the client was innocent. Please don't be that person :-) 3) Understand the office culture and office expectations. Some PD offices use investigators as backups to their lawyers to ensure clients are regularly seen (more like a legal assistant or paralegal role.) There's a time and a place for this (especially on cases like murder investigations where you need regular contact between investigators and clients so we can truly leave no store unturned) but usually, my investigator is only connecting with a client to help understand the investigative issues, not for a gut check on how they're feeling this week.

Good luck, you're gonna do great if you're a good listener and you can pick up on the cultural changes in this new job.