I recall that when Robin Hood: Prince of Thieves (1991) was released in 1991 a lot of people complained that the star in the title role, Kevin Costner, didn't speak with a British accent. Of course, a few centuries ago the typical accent in England was probably as close to a modern American accent than any of the British accents we hear today, and Robin Hood, if he existed, might well have spoken Norman French at home (if he was a Norman nobleman as in some tales) and some form of early Middle English with the Anglo-Saxon common folk.
But in movies we have come to expect that everyone from medieval knights to ancient Greeks and Romans to characters in a fantasy universe (as with most of those in HBO's Game of Thrones) speak with a British accent appropriate to their socioeconomic status. Here's Bill Maher having some fun with that:
On the other hand, American accents have become standard for some purposes. For example, I recall correctly, most of the actors on the television series Hercules: The Legendary Journeys (1995-1999) and Xena: Warrior Princess (1995-2001) had American accents even though both series were produced in New Zealand and many of the actors were from there or Australia. For that matter, many famous Australian actors such as Hugh Jackman, Chris and Liam Hemsworth, Cate Blanchett, Nicole Kidman, Mel Gibson, et al are much more often heard speaking with an American or British accent than with their native one.
Of course, if a character in an English-language film is supposed to be speaking another language ancient or modern or invented, it makes sense to have the character speak in English, and then they have to use some accent. There's no such thing as unaccented English. (My father once asked someone he met in Paris if he was Australian, and when the man replied that he was, my father said, "I thought I recognized the accent," and Australian smiled and said, "I haven't got an accent, you've got an accent!") What's curious is that German characters supposedly speaking German, or French characters supposedly speaking French, will sometimes speak English with a German or French accent, which if you think about it is a bit odd.
Even odder, in the film Moon Over Parador (1988) Richard Dreyfus plays an American actor working on location in a fictitious Latin American country who has no problems pretending to be the country's dictator, whom he happens to closely resemble, even in private conversations with the dictator's friends and acquaintances. The implication is that the residents of Parador don't speak Spanish, they just speak English with a Spanish accent.