Because of the complicated effects involved in making Xerxes a 10 foot giant, Rodrigo Santoro filmed almost all of his scenes alone in a green-screen room and had little/no interaction with the rest of the cast. He said it was difficult but he got used to it as he had also done it on the first film.
Rodrigo Santoro spent 6 hours in the make-up chair every single day to morph into his character. Even after the make-up was done, he couldn't move much because all of the jewelry and piercing that he was wearing was very delicate and would break if he moved a lot.
Though a large part of the film takes place on water and boats, the film was filmed completely "dry" with all the water digitally added in later. Director Noam Murro said this allowed great freedom in filming.
Sullivan Stapleton thought Gerard Butler looked "really huge" in the original 300 (2006) and that he would similarly have to bulk up and get a lot bigger as he was playing the lead role this time around. But he was actually asked to slim down because he is playing an Athenian and not a Spartan, he was at 10-12 kilos below his usual weight. All The Athenians in the film are also shown as having "six pack abs" like the Spartans but the Athenians are not as big.
Sullivan Stapleton was surprised by how wordy the film was. He thought he would "just be roaring and chopping off limbs throughout" but actually has a great number of dialog scenes along with fight scenes.
The original script featured King Leonidas from 300 (2006), but he was ultimately cut out. This was due to Gerard Butler, who turned down to reprise his role as Leonidas, since it "wasn't really [his] thing".
Eva Green revealed in an interview, that she underwent major preparation for the role. "I trained four hours a day with a group of stunt men from L.A. for three months before we began shooting. And then I continued training and sword fighting while we were shooting". She added that the training "was extremely liberating and empowering" and that her character of Artemesia is "the strongest, toughest, bravest character I've ever played".
The movie was initially going to be about the story of Xerxes and his military expeditions. The story and narrative were changed significantly and the script was revised in subsequent drafts to have include the Athenian leader Themistocles.
Producer Gianni Nunnari said that there were a few big names who were interested in the lead role of Themistocles but they decided that rather than a star vehicle they would make is a star-making vehicle - much like the original film made a global superstar out of Gerard Butler - and cast Sullivan Stapleton in the lead role.
The entire male cast of the earlier film, 300 (2006), famously sported extremely well-defined "six pack abs" as they went bare-chested for the entire film. The look, which came to be known as the 300 look or the Spartan look, became very famous in the media and popular culture and came to be strongly associated with the film. The look was heavily promoted by the marketing department in posters, stills, behind-the-scenes features and footage and by the male actors in interviews where they spoke about the rigorous training required to achieve that look. For the sequel, Sullivan Stapleton said that he also had to develop "six pack abs" as it was almost a pre-requisite for a "300" film.
The film is credited as being an adaptation of Frank Miller's graphic novel 'Xerxes'. However, it is an adaptation in name only. The script was written at the same time as the novel and not after the novel was finished so ultimately the two are different. In fact, as of the film's release date, the novel is not even complete and Miller has reportedly drawn only 2 out of a projected 5 issues and none have been colored. The book has no release date yet or an expected date of completion either.
Sullivan Stapleton said that he was extremely skeptical when he was cast whether he would be able to build the requisite "six pack" for the role. He thought some people naturally had it and some people didn't and he was one of the latter group. But once he went through the training, he got "six pack abs" himself and said he was pleased to realize that anybody could develop them with the proper training.
The film was previously called "Xerxes", the title of the Frank Miller graphic novel on which the film is based, and later "300: The Battle of Artemisia", the title used during filming. Ultimately, the present title "300: Rise of an Empire" was used because the other two were deemed too exotic.
Director Noam Murro said that the instant Sullivan Stapleton came in to the room to audition for the lead role, he knew he was "the only choice for the part". He told the producers that he would hire no one but Stapleton.
Sullivan Stapleton was filming the television series Strike Back (2010) while preparing for the film. So along with filming every day, he had spend 3 hours training - 1.5 hours in the gym lifting weights and 1.5 hours sword training to get the stunts right. He was extremely exhausted at the end of the day.
Gym Jones, who burst into the spotlight for the physical training they provided to the male actors in 300 (2006) were again employed to train the cast for the sequel. They trained the cast before filming began and also throughout filming so that the cast could attain and maintain the physical shape required for their roles. The return of Gym Jones for the sequel was due to their long standing relationship with Zack Snyder who wrote and produced the sequel.
Zack Snyder conceptualized the project and wrote the screenplay along with his 300 (2006) writing partner Kurt Johnstad and was on-board to direct but dropped off due to commitments to Man of Steel (2013). Noam Murro was picked by the studio to direct but Snyder still produced the film.
Sullivan Stapleton revealed that this was a role which absolutely required him "to get a six pack for the first time in my life" and he reportedly adopted a diet of almost 100% yogurt for the duration of the shoot to maintain it.
In reality Sparta was a land power and the Spartan Navy at the time of the Persian Wars was feeble. Sparta only contributed 16 ships to the combined fleet at the Battle of Salamis. The closest naval rivals to Athens at this time would have been Corinth, Megara and Chalcis.
Hans Matheson had auditioned for Zack Snyder for a role in his film Sucker Punch (2011), he however did not win the role. When Snyder called him again to audition for this film, Matheson readily agreed and won the role this time around.
(at around 1h 19 mins) The quote "Better we show them, we chose to die on our feet, rather than live on our knees!" is a paraphrase of a rallying cry originally spoken by Emiliano Zapata, during the Mexican Revolution of 1910.