Yes but this places a task on someone, to decide when they're open to be interrupted or not. This places that work on them, and makes them do this work whether

Yes but this places a task on someone, to decide when they're open to be interrupted or not. This places that work on them, and makes them do this work whether or not someone wants to interrupt them. In the office scenario, I as the interruptor take on the burden of finding out when someone is open to being interrupted (and it's not that hard). For example I can just say "I'll ask him the next time I see him" and wait until I see him go to the coffee machine or the bathroom. Or, if it's more urgent, I can take a peek in his office and see if he seems to be engrossed in thought or more relaxed.