Uhmmm good question. An actor's training includes "freeing" their voice and breaking out of their usual speaking habits. This gives them a lot of flexibility in the way they speak, which includes using other accents. So there's a bit of groundwork to do before they have a "talent" for accents.
Some of the work behind using different accents is simply listening to how your own speaking habits differ from the target accent. You could literally list the differences, and then work on replacing your habits with new ones. That would be a start. :)
Actors today need to be able to adopt whatever accent the role demands. In simple terms, the accent which we hear is the accent of the character, not of the actor. When an actor is preparing for a role, they take on a new persona, which involves adopting the body language, tone of voice and accent of the character they are playing. This is especially the case with historical and fantasy dramas. Actors often work with professional dialect coaches to train them in regional accents.
The UK has a very complex and multi-layered landscape of accents reflecting not only geography, but often also the social class, status, education and age of the speaker. The accents of the fictional country in Game of Thrones mirror this to a large extent. Regardless of the origin of the actors, the nobility speak in RP accents, while the lower-status families have regional (mainly northern) accents.
NB The exceptions here are:
- Sean Bean. He acts in his own gruff Yorkshire accent.
- Peter Dinklage. His American pronunciation comes through, so he sometimes just sounds weird.
Hi Natalia! No answer to your question (I'm just looking forward to learning what other people think, as well), but rather a couple of funny videos on it :))
https://www.youtube.com/watch?v=OHegldrHEHI - The Lannisters' accents. Scale of poshness inside :)