in the cast lists, each person is listed with their real name and the character name. In some of the lesser roles, why is there sometimes a character's role name followed by their real name in brackets e.g. Head Laborer (as David K.S. Tse)?
That doesn't really have anything to do with it being a lesser role. In some movies even the top billings can have the attribute "(as Some Name)" set in the IMDb profile, like for instance Arnold Schwarzenegger in his first movie, Hercules in New York (http://www.imdb.com/title/tt0065832/c...), where it says "Hercules (as Arnold Strong 'Mr. Universe')". In the case of David Tse, his birth name seems to be David Ka-Shing Tse, and during his career he has been credited both as David Tse, David Ka-Sing Tse, David K.S. Tse and David K. S. Tse, depending on the movie/series. David Tse is the name he has been credited with most, his common name if you want, so IMDb uses that name for his profile. But movie and TV-series credits are sought to be as accurate as possible, so if an actor is credited in it with a different name than the common name, they add the attribute (as Credit Name) in that profile.
So the parentheses doesn't reflect on the role, but on the actor playing the part, and simply shows the name the actor was credited with in the credits of that movie.
It doesn't only apply to actors either, but anyone credited in a movie/TV show. For instance Steven Spielberg is credited as Steven Spielrock in The Flintstones (http://www.imdb.com/title/tt0109813/c...), where he was a producer. So that field is used whenever the name in the credits does not match the main/common name of said person on IMDb.