20 Things You Didn’t Know About Gladiator
5. Period Inauthenticity, Part I: Thumbs Up Or Down?
That picture that Walter Parkes used to sell Scott on making the movie was called 'Pollice Verso', which translates as 'Thumbs Down'. That may or may not have been the correct interpretation of that gesture, however.
It's commonly believed that Roman officials - mostly, the Emperor - would spare lives with a thumbs up and order them ended with a thumbs down. Historically speaking that isn't correct: the thumb gesture actually represented a sword's thrust - up meant death, while down meant sheathed.
Gladiator's cast and crew were fully aware of this widespread misconception, but chose to ignore it, for the same reason that Crowe's request that his Spanish character should speak in a Spanish accent was turned down: the audience expected something else, and giving them an authentic detail instead would only confuse matters and disrupt their suspension of disbelief.
That's why Commodus turns his thumb down to execute gladiators in the arena, and that's why Maximus doesn't speak like Antonio Banderas.