Can I set up a screensaver on my MacBook's display while I am using a connected external monitor as my main display?

I have an external monitor attached to my MacBook Pro. I only use my MacBook's display when I have an online meeting, or need to read a document on a second screen while working on a project on my external monitor. Because I don't use it much, I'd like to have a screensaver running on the MacBook's display - is that possible, or will the screensaver never kick on because I'm using the HD?

Secondary question: If I'm not able to have a screensaver on my MacBook's display while working on my external monitor, I may decide to use my MB in clamshell mode. Is it bad for the laptop to always be in clamshell mode, and should I get a laptop stand with a fan/cooling features?