When you have to make a movie with just a couple of characters, a mobile phone, a car and landscapes, you have to put the effort in the dialog.
I found the dialog and reactions of both leading actors quite natural and credible. We are knowing the characters little by little, changing our view about them as the movie goes on.
Dialog is also witty as the two characters are quite intelligent and has a lot to say. I could not help smiling and even laughing in several moments... I have also found myself thinking during and after the film.
I do not know about accents, as my mother language is not English. So I do not know if the English they speak it is good or bad or about the quality or accuracy of the accent (I understand who complains about this as if the movie was in Spanish, trying to depict wrongly people from this or that area, I think I could find it annoying)... I do not know if it is 100% original or there are other movies like this... all I can say it is that I found this "small" movie really entertaining, it surpassed my expectations and I can only strongly recommend it.