Hi,
indeed, I don't understand the purpose of group by after select distinct with the same field.
if you need the set of unique Actors in the column, there are many way to do it. I think faster way is (don't works with primary field, you need to duplicate it) - click 'edit field', choose single select without clicking 'Save', under 'colored options' you have a list of distinct values with scroll bar, you can copy this list to clipboard, and press 'Cancel' instead of 'Save'.
Then use list in clipboard according to your needs.