Hi Eville Geek,
Thanks for reaching out in Apple Support Communities. I understand that you're trying to use the Character Viewer in Mail on your iMac, but the pop-up window doesn't appear when you click on it. I know it's important to be able to use this feature in Mail; I'd like to help. I'd recommend starting with the steps below first:
If the issue persists, use these steps to start your iMac in safe mode and test for the behavior there: Use safe mode to isolate issues with your Mac
After testing in safe mode, restart your Mac normally and test again. Let me know what you find.
Have a great weekend!