To MBCXHARM (post #5)
When you open the speed window, the thrust levers do not spring to life because the A/T would be more accurate with speed window open than with FMC in control.
In Cruise mode, the autothrottle is in a lazy mode, so as to minimize thrust changes. Thrust is kept constant as long as the speed, averaged over a certain time, is at the commanded value (this is a moving average). The instantaneous speed value may be off the target by quite a margin, without the A/T reacting.
When you change the target, either by changing the FMC target, or by opening the speed window, you will reset the moving average calculation, so any instantaneous deviation from the target speed will cause A/T reaction. Leave the speed window open for a long time and the same lazy behaviour may show up again.