I was put in a similar situation today, and even though Ive planned out what to do and how to properly act, I still fucked up and could have been in the same situation Castile was in. I got pulled over for a regular traffic stop. Keys out and on dash, hands on wheel. Officer walks up and we start discussing why I was pulled over. He asked for my license and I fucked up and started reaching for my wallet before telling him. While pulling my wallet out he asked If I had any weapons in the car, I said no (truthfully, was not carrying at the time) but I do have my carry permit.
If I had been carrying, slipped up and said yes, while pulling my wallet out, without informing him I had a legal carry permit fast enough, I could see a similar situation go down. Despite everything I had planned to do for a stop, I still slipped up. At this point, I would be way more comfortable being ordered out of my car with my hands up while they sit in the cruiser and tell me orders over radio for every future stop. Less of a chance of them mistaking my actions for a draw and both of us go home safe.