7. Brad Pitt - World War Z
The Commitment: For the part of the movie where Pitt's character Gerry Lane lands in Russia and is subsequently enslaved and forced to fight the zombie horde for the motherland, Pitt fasted to the point of emaciation. Pitt then spent the grueling months of the Budapest shoot (Budapest standing in for Russia) scruffy, bearded and significantly malnourished. It can't have helped that creative disagreements were a constant niggle during this period.
The Result: Seeing as Pitt was both producer and star of World War Z, his unnecessary commitment for the film was two-fold. Though the Budapest shoot originally constituted almost half of the film, none of the footage ended up being used in the final cut, meaning Pitt sank money into the project and went considerably less handsome for shoots that ultimately no-one saw. Instead, an alternate ending was shot months later, with the stench of bad publicity seeing journalists prematurely declare World War Z a failure. Thankfully for Pitt, the film was a success, though his hobo transformation was ultimately for naught.