I actually found an easy workaround for now if you want to create a Smart Collection for just a single actor if you know at least 1 movie they are in:
- Click on a movie with that actor
- Click on that actor in the cast list
- This should bring up a list of the movies they are in, but with no options to save as a Smart Collection
- Add &advancedFilters=1 to the end of the address in the address bar to convert this to an Advanced Filters search
- You can now save this as a Smart Collection
