We did something really similar to that. It can be very difficult to try and talk to an actual sister and trat her as a PMN, even if she is acting like a PNM. No matter how familiar we would start getting in practice conversations, the older members would always stay in character. Practice conversations are great, but not if they dissolve into friendly chit-chat between sisters, which happens very easily.
Make sure an practice both positive and negative situations, or people might begin to think that only horribly acting PMN's go through recruitment!
Also, make sure to have back-up questions ready for situations with conversation lulls.
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