If you get stuck, get away from your desk. Take a walk, take a bath, go to sleep, make a pie, draw, listen to music, meditate, exercise; whatever you do, don't just stick there scowling at the problem. But don't make telephone calls or go to a party; if you do, other people's words will pour in where your lost words should be.
It's really hard to get a coffee with someone. I have to call my agent, my agent calls their agent, their agent calls their manager, the manager calls back, the actor sends someone to the manager... then you get, 'Yeah, yeah, I'd love to have dinner at six,' and all I wanted was coffee! It can take, like, six days to get coffee.