An ex-colleague of mine commented to me that casting Brad Pitt as Achilles and Orlando Bloom as Paris in the movie Troy seemed odd. Well, it kinda makes sense when you think about it this way:
Achilles: sulks in tent over a woman for most of the Iliad, then goes on massive killing spree. Think Brad Pitt in "Legends of the Fall", and you see how that might fit.
Paris: you need an actor handsome enough to convincingly steal the world's most beautiful woman from her husband. Also, experience with a bow and arrow would be useful (must be able to hit an exposed heel from a distance). Hmmmm ... who would we pick? = )
It makes sense for the other roles as well:
Priam: Elderly, dignified King, with one emotional scene where he begs Achilles for his son's body. Naturally, we need an elderly Brit actor with a dignified demeanour, who can convincingly break down and cry. With dignity. Well, since Gielgud's death, as well as Harris's, we're really only left with O'Toole. Or Hopkins. I'd pick O'Toole.
Hector: A one man Trojan killing machine. Let's face it, Crowe, or even Jackman, would fit the bill but they've played these types of roles before before. Well, Bana has as well, but technically it was a CGI Hulk, and Bana is Australian, which seems to be a criteria for playing these brutish masculine roles.
Odysseus: Now this is a puzzler. Sean Bean consisently plays lower-middle class/commoner outsider anti-heroes or villains (think Sharpe, the NCO-turned-officer, 006 in Goldeneye, or the Irish terrorist in Patriot Games) who usually dies (Boromir). Odysseus doesn't die in this one. The only way I can make this work is by rationalising that Odysseus doesn't fit in with the rest of the Achaeans, because his brain is capable of thoughts other than "kill!kill!" and "ooh, that woman look good - me launch ship now".
Helen: well ... as long as she looks good, everything else works out, right? = ) Btw, here's a link to a miniature of Helen of Troy, from Redoubt Enterprises' Trojan Wars Line - someone should tell Redoubt that it was the face that launched a thousand ships ... =)