I don't really know what I'm doing, I've fixed a car once, previously.
A gentleman with a code reader says I've got a coolant sensor problem (more precisely, "#22 22 Open or short circuit with coolant temperature sensor circuit" as described here: http://www.toyoland.com/repairs.html
So I've just bought a new coolant temperature sensor and am trying to replace my old one.
Chiltons and Haynes both tell me I need to drain the engine of coolant.
Is that entirely necessary? Given what appears to be the location of the sensor, it looks like (and again, I don't speak with much experience or authority) I can replace the thing without doing that. One of the silly reasons I want to think I don't have to drain coolant is because the gentleman with the code reader knows I don't know what I'm doing, that the first thing I've ever done to a car was clean the IAC, and he still talked like it would be a simple procedure for me to get the part, go home, and replace the coolant sensor.
I'm fixing what appears to be a common stalling-while-hot issue. In addition to responding and thanking to/for replies here, I'll eventually post a narrative of my whole series of adventures with IAC, EGR, MAF, etc, when I really think I've got the problem solved.