The thumbs-up/thumbs-down gestures which stem from gladiatorial events had different if not opposite meanings to what they do today. A down-turned thumb by an official symbolized the winner burying his sword in the sand and the loser's life spared; an upturned thumb expressed delivering the killing stroke, symbolizing a slit throat.
The actual gesture remains unknown - our belief in the thumbs-up/down stem from a painting by Gerome (pollice verso - with turned thumb) the surviving descriptions 'Infesto pollice' (with hostile thumb) and 'pollice premere' (with thumb pressed down) are too ambiguous to conclusively state what gesture was used in reality.
Director Paul W.S. Anderson said that he was not pleased when a lot of people online said that Kit Harington's six-pack abs were digitally enhanced and not real after the first stills and trailers for the movie were released. He reiterated that they were completely real with no make-up or digital enhancement and Harington actually looked like that for the shooting of those scenes.
Kit Harington suffered from body dysmorphia (excessive anxiety about the appearance of one's body) while working out for this role. He said he became completely obsessed with it to the point where he was "going to the gym three times a day for six days a week." He suffered from a case of "exhaustion" and eventually his trainer stepped in to rein him in. But he said that he was "proud of what he achieved in the end."
The peculiar form of hand shake Severus and Senator Corvus share is real and period accurate, recorded on old frescoes and statues.
Kit Harington's shirtless introduction scene (in the gladiatorial arena in London) was shot on the last two days of principal photography to give him as much time as possible to get his body in shape. He started training before filming began and throughout filming to peak for that scene.