Liam and Ralph play king of the gods Zeus and Hades, god of the underworld, respectively, in the mythical blockbuster. The pair became good friends on set and had a hard time being serious in their scenes.
In a joint interview on British morning TV show 'Daybreak,' Ralph said: "It's a romance, a Godly bromance. We go in very serious - like we're going to do this - and then we catch each other's eyes."
Liam added: "All these fake beards and long hair and costumes and stuff - you have to have a giggle."
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The two actors also spent their time on set imaging who would win in a fight between two of their most famous characters - Ralph's Lord Voldemort from the 'Harry Potter' films and Jedi Knight Qui-Gon Jinn, who Liam played in 'Star Wars Episode I: The Phantom Menace.'
Ralph said: "He (Lord Voldemort) wouldn't stand a chance. The thing is, if Voldemort has wand power - you're finished. But once I get in a reach of these huge arms, then I'm done for."