What if you keep finding yourself reminding someone that you're on the same team because they're acting like you're on the opposing side?

My answer to that is to ask the 'opposition person', "Are we on the same team? If we are, then I need to have you refresh in my mind what our goals are and how we're going to achieve them." With that said, the person (should) be able to explain why they are not supporting the cause OR: what they see as being most important to achieve the goal and then details can be worked out adding to 'the general plan' OR: showing the person why their plan needs to be modified to succeed and put them to task to figure it out.