Chris and I were hoping to catch either the 8pm or the 8:30pm bus this evening, from La Paz to Potosí. But when we got to the bus station, at 6:30pm this evening, the seats on both of these buses were completely sold out. However, they did have two seats left (the last two) for the bus to Sucre, which left — well — immediately, at 6:30pm! So we immediately bought the two final tickets, ran to the bus, whacked our luggage underneath, and jumped on. And that's how we ended up going to Sucre tonight, instead of to Potosí.
It was pretty damn crazy, I gotta say. In true Bolivian style, as soon as we bought the tickets, the guys at the desk said: "quick, under here — we haven't got much time!" And by "under here", they meant "crawl through the hole that you're meant to put your checked baggage through, it's the quickest way to the bus". This was fine with us: it meant that we got straight on; and I think it let us avoid whatever stupid "exit tax" we were meant to pay, upon leaving La Paz. And literally as soon as we jumped on the bus, the doors closed, and we were off. At least there was no waiting around — quite the opposite.
Anyway, the 12-hour night bus from La Paz to Sucre was pretty uncomfortable, and on a pretty bumpy road; but at least we got there in the end. It was supposedly a "bus cama" (i.e. "bus with beds for seats"); but the Bolivian idea of this is slightly different to, say, the Argentinean idea of it (Argentina is famous for its super-luxurious buses). The seats went back slightly more than usual, and they were slightly bigger than usual; but they had really uncomfortable plastic covers, and there was too little leg room for you to actually stretch out and sleep properly. Most regular buses that I've been on have been more comfortable. Anyway, in Bolivia, you take whatever you can get, as far as inter-city buses go.