I had to do my 135 PIC initial with my CP and POI to certify the CP as a check airman. It wasn't really too big of a deal, but it was very thorough. We covered everything in the 135 IFR PIC modules, and beforehand the POI said it would basically be an ATP checkride. That comment made me nervous, but since I'd already been doing a bunch of part 91 flights in that aircraft (Chieftain) it went well.
Just make sure you're fresh on any regs that may come up, and that you have a thorough pax briefing (unless you only haul boxes of course). Good luck!