That comes down to handling every employee as an individual and knowing what they do and don't respond to. If you've got someone who doesn't take "I need X" as a request to do X, you just adjust your phrasing so that your meaning gets through.

How about "I need X and the only person who can do it is you"?

It is interesting how this changes the tone.

"You will do this" no agency, no control, no considerations, skips calling out authority and goes direct

"Will you do this?" Direct, has agency but potentially confrontational if you say no

"I need you to do this" indicates their authority, direct, no control, could be inconsiderate to some as they overriding your priorities for their own

It can to be softened up with a question but if they really need you to do it I feel like your response also softens it up.

I think adding a acknowledgement of inconvenience, reason, urgency or explanation to it also helps.

"Hey Jim, I know you are working on X and this will be inconvenient for you but I need you to tackle Y" ideally a reason or compliment would be good too.

"I need you to do this because you have the most expertise on this and feel no one can do it well but you"

"I need you to do this right now. We are losing hundreds of thousands of dollars with the web server down" I feel this is an acceptable reason to demand to interrupt someone to take care of something that has a major impact on everyone

And for the life saving measures saying "I need you to evacuate this building immediately" is completely acceptable to me with no reason needed. Being butt hurt over a demand versus a kind request is better than dying.

Haha yes, "I need you to evacuate" is certainly no good -- but "evacuate; this building is burning and you will be trapped soon otherwise" is much better than "evacuate". People who grow used to urgent-sounding requests are likely to blow them off.

I once got fired for not doing something the way the boss wanted, because at my previous job we did it the opposite way. He had explained that he wanted me to do it, but didn't say why, so I didn't remember. I too find that when I ask someone to do something, they're much more likely to do it right if I explain why, not just how. (Getting the method wrong is different from ignoring the request, but they both spring from the same underlying phenomenon -- we are likely to discard information if we don't know why to heed it.)

Yeah I think you boiled it down well that interruptions are often abused and so people become desensitized to meaningful levels of urgency.

Context matters a whole lot, some of which doesn't even have to be verbal, but I've found that adding the why goes a long ways in getting people on board because no one wants to dive head first blindly into a demand or request without knowing the reason or desired outcome.