Well, a "Dialect Coach", originally called a "Dialogue Coach" back in the early days of "talkies", nowadays helps actors by coaching them in whatever accent or dialect is appropriate for their rôles.

Actors who made the transition from silent movies to the talking pictures often needed help with their previously unheard voices: either reducing a regional accent, providing a more acceptable register or, most often, helping them deliver their lines. These coaches were known as "Dialogue Coaches" simply because they helped with dialogue. Many years later "dialogue" has come to encompass all and everything to do with the spoken word in movies. Some coaches who do this work choose the title "Dialect Coach" or "Accent and Dialect Coach." To compound the confusion, some Dialect Coaches are also qualified voice teachers and may work solely with an actor's voice, working on breath, sound and tone. In Europe, a person working in this capacity is known as a "Voice Coach." In the United States a "Voice Coach" is someone who coaches singing.

Point to note: anyone can call themselves a Dialect Coach, there are only a few in the world who have the ability and experience to teach (almost) any actor to speak in (almost) any accent.  

Here it should be noted that the word "actor" is used to mean actors of either sex throughout this website.

For the answer to the second question, please see Biography.

Why have accurate accents? Why not just allow the actor to speak in his or her natural accent?  

The suspension of disbelief dictates that the words must be spoken as if the character were genuinely from the relevant area and time period, as appropriate. For example, modern North American accents do not fit into Charles Dickens' London.

What makes a really great Dialect Coach?

Andrew Jack's background as an actor in film and television is invaluable when it comes to relating to how an actor is feeling at each stage in a production: elation at being awarded a role; anxiety - am I going to be able to do this? tension - first night in the theatre and first day on a film set can be stressful or even nerve-wracking and are always highly demanding for any actor. Andrew has worked on over 50 feature films and there is little in film production with which he is unfamiliar. Andrew's broad knowledge of theatre is indispensable; his first experience of television was at the age of 13 when he was in a TV sit-com called "Whack-O" with Jimmy Edwards (more).