I just went to IMDB and looked up Alexander and here is the cast list of all the major players. It looks like Seleucius isn't in the movie at all.

Colin Farrell .... Alexander
Angelina Jolie .... Olympias
Val Kilmer .... Philip
Anthony Hopkins .... Ptolemy
Jared Leto .... Hephaestion
Rosario Dawson .... Roxanne
Gary Stretch .... Cleitus
Jonathan Rhys-Meyers .... Cassander
Ian Beattie .... Antigonus
Brian Blessed .... Leonidas
Francisco Bosch .... Bagoas
Bin Bunluerit .... Poros, King of India
Raz Degan .... Darius III, Shah of Persia
Michael Dixon .... Dimnus
Paul Hornsby .... Machatus
Neil Jackson .... Perdiccas
Christopher Plummer .... Aristotle
Garrett Lombard .... General Leonnatus

Also here is a bunch of trivia related to the movie also from IMDB,

Some scenes from the film were supposed to be shot in India but the Indian elephants weren't well trained so they had to shoot it in Thailand instead.


'Colin Farrell' sprained his ankle after falling down a stairway while filming.


Val Kilmer gained over 50 pounds and stopped working out for the plump role of Philip.


Historical advisor and Oxford University professor Robin Lane Fox doesn't get an on screen credit. His price for giving his advice was to be allowed to take a place at the head of what is one of the largest cavalry charges ever filmed. Professor Robin Lane Fox was used to riding around the English countryside but gladly dressed up as a Macedonean cavalry officer to live his dream of Charging for Alexander.


The biography of Alexander by Robin Lane Fox was an original inspiration and source of information to director Oliver Stone.


Brad Pitt was considered for the role of Hephaestion, but turned down the role due to personal concerns with the character.


'Oliver Stone' previously tried to mount the project with Tom Cruise in the title role.


Sean Connery was originally considered for the role that was eventually offered to Val Kilmer.


The movie was originally scheduled for release on 5 November 2004 in the USA, but was moved to 24 November 2004. Studio executives stated that the move was to put it in time for Oscar consideration.