There is more, if the thermostat gasket is weeping ( you won't always see liquid ) the bolt threads will become corroded / stuck to the hole threads.

If the bolts are in through holes, wire brush where they stick out, use penetrating oil and _let it soak_, it didn't corrode in 30 seconds and anything you spray on won't soak in that quickly either. Tapping on the bolt head helps to wick the oil somewhat.

A bit of heat on the aluminum helps as well ( like heat gun .) Start to turn the bolt, it is hard to describe what a "I'm stuck going to twist off" bolt feels like so I can't help there. If the bolt starts to turn then gets tight, stop / reverse direction but don't turn until tight, try to remove again but stop before the bolt becomes difficult to turn. What you want to do is use the free play and work the bolt back and forth breaking free corrosion. However, this leads to fatiguing of the bolt and it may break off where removing in one shot ( still hard to turn ) it wold survive. Removing stuck bolts is somewhat of a black art. Removing broken stuck bolts full on dark side.

Sometimes there is no way around breaking a bolt or damaging something. Remember broken bolt extractors generally have less strength than a full size bolt. The _ONLY_ broken bolt extractor worth purchasing is made by Ridgid, it is a straight spline 2 to 3 inches long and a nut to drive it. This gives you the ability to turn in both directions ( remember the "free play" to work it back and forth breaking corrosion )

I've even gone so far as to cut housings away to relieve through hole corrosion / gain access to the top portion of threaded hole.