I know it's rainy season, but it it typically the kind of rain that will storm for awhile (20 minutes to an hour) and then be sunny for a couple of hours, followed by another shower? Or is rain all day/dark skies the norm in November?

November is by far the wettest month on Samui – expect rain most days, and it can be quite intense – flash flooding, rough seas and infrastructure interruptions.

For example, the average rainfall in November is over 400 mm.
In Phuket, at the same time, it is 120 mm.

The rain peaks at the beginning of the month – this is because it is the turnaroundperiod from the southeast monsoon to the northwest. The Northwest picks up moisture from the Gulf and deposits it on Samui and the southern Gulf coast.

by the end of November it eases a little, and by the end of december the dry season kicks in.
Average rain on Samui in December is 150 mm, Jan is 90 mm and Feb is 30 mm, so you can see how comparatively wet it is in November.

Thank you. I also checked actual weather records from last year and it is not only wet but also mostly dark skies.

Varies from year to year—when the sun comes out, it is, of course, very strong, and things dry quickly.

Weatherwise in November, you're much better off on the Andaman coast or the Eastern Gulf.