The Toronto Sun (and likely other members of the Sun chain) has a feature on bad movie accents in 20 or so movies, and Brangelina form a Classical duo at numbers five and six:
WHO: Brad Pitt as Achilles. MOVIE: “Troy” (2004.) ACCENT SUPPOSED TO BE: Ancient Greek (which, in most Hollywood movies – becomes a British accent). SOUNDS LIKE: American cowboy with a hint of English.
[…]
WHO: Angelina Jolie as Olympias. MOVIE: “Alexander” (2004) ACCENT SUPPOSED TO BE: Ancient Greek. SOUNDS LIKE: Possibly Russian. Also odd? Co-star Colin Farrell forgoes the Greek completely and speaks with a British accent instead, making Jolie’s even more out of place.
Back when Troy came out, Language Log had an interesting post on Pitt’s enunciation:
… Jolie doesn’t seem to have merited similar treatment (other than a zillion reviewers mocking her for her Russian/Transylvanian/East European affectation)
rogueclassicism 
Colin Farrell not British accent, Dublin boy.. and fair play to the Irish actors who kept their accents in this movie