She was required to wear saloon girl- style costumes as seen in the Wild West for the movie, which meant that corsets were a must for authenticity.
However, Megan appears to be something of a sadist, as she told the Sun she was happy to have them tightened up as far as they would go.
Her waist is normally waspish, but the former Transformers actress told MTV that she succeeded in getting it down to 18 inches thanks to a lot of string-pulling - ouch!
For those of you who don't know the story, Jonah Hex is based on the DC Comics of the same name and features a surly bounty hunter whose face is terribly scarred.
Jonah will be played by Josh Brolin in the forthcoming movie, which sees him on a quest to protect the innocent.
However, if you want a more instantly recognisable outfit, Captain America superhero fancy dress could be good to choose as this is also being made into a film.