I reckon more or less that the core instinct in friendly fire is fear. It's something scary when homies turn hostile or even just have a gun. What can be done to prevent friendly, strategically wasting assets, and even on the squads!
If more comfortable discourse would be the only thing that would be quick but you risk lulling senses by trying to be comfortable, I'm talking about gaming. In real conflict it would probably be enough. But still it would be miserable to chicken soup a teammate and watch him sleepily die.