Non-Cantonese speaking actors

In HK flicks, they usually have the non-Cantonese-speaking actors acting in their own native languages. For the final product, they have Cantonese actors dub over the "foreign" lines into Cantonese. You therefore have a movie where everyone speaks Cantonese.

Nowadays, s lot of Hong Kong movies employ a lot of Mandarin-speaking actors. On the finished film, if you watch the Cantonese version, you hear all the Cantonese actors in their real voices with all the Mandarin actors speaking a dubbed-over Cantonese. Similarly, if you listen to the Mandarin track, the Cantonese actors have their voices dubbed over by Mandarin speakers while the Mandarin actors' real voices are heard.