You suddenly find yourself needing to research something that has nothing to do with your current work – the exact time it would take to go between London and Paris on foot, if one could walk on water, say, or perhaps the airspeed velocity of an unladen swallow.
Things no one could have paid you to read or watch, are suddenly fascinating and you say something like “Wow, Oxford dictionary, what plot, what verve. I wonder why I never read it cover to cover before.” Or perhaps “Paint drying. Incredible!”
Your house needs to be clean. I mean, really clean. Why did you never think to wax the floor of the front porch before?
If you actually make it to your desk, you realize your keyboard is filthy and must be unplugged and cleaned, with qtips and paint brushes.
What the block means, at least in me – I am almost exclusively an intuitive writer. This doesn’t mean it’s unplotted. I plot and pay a lot of attention to structure. It’s more that I don’t know what I’m trying to say with the work till I read it. Meaning leaks in.
Usually what brings me to a complete stop is knowing that meaning went awry in the plot, sometimes. But it can also be sheer lack of self-confidence. With Darkship Thieves, I moaned and b*tched for MONTHS because the plot felt too fractured to me. With Darkship Renegade? Honestly? I think it’s because I WANT it to be SO good that I’m blocking myself.
The funny and sad thing is that without these fits, I could easily write a book a month. (With necessary revisions.)
The good news is that I think I’m almost done with this block thing this time. Now, if real life doesn’t interfere, I should be able to finish. (Yes, I thought that before, but real life intervened. So, keep fingers crossed.)
According To Hoyt 
I vaguely recall that a famous golfer wondered, in the middle of a tournament, how long it would take to walk straight from there to New York – and took off walking to see, at least till they locked him up and forced treatment on him. Tim O’Brien, on the other hand, wondered how long it would take for a US platoon to walk from the jungles of Viet Nam to Paris, and wrote the novel “Going after Cacciato.” I guess that defines one of the few differences between schizophrenics and writers.