I believe [Actors]=Bruce Willis does not require setting the field as a default search field? That's for when you type just "Bruce Willis".
Technically it should be [Actors]="Bruce Willis" or [Actors]=[Bruce Willis]. Note that this will not find "Willis, Bruce" or other variations.
Maybe try using the Wizard to build the search query or to see what it's converting the text to.
Edit: OK, with [Actors]=Bruce Willis (without quotes) it may fail when Actors is not a default search field because it's actually searching for [Actors]=Bruce AND Willis, ie, files with Bruce in [Actors] and with Willis in any of the other default search fields.