Biopics have been a cinema institution since 1900, when French actress Bleuette Bernon starred as Joan of Arc in George Melies silent film of the same name, but such a role comes with far more pressure than usual for the starring actor. The portrayal needs to remain true to the character as the audience already knows it, and this requires something that even the most talented of actors can struggle to bring to the screen: authenticity. The job, then, becomes even more of a challenge when it comes to biopics of figures who are in the acting profession themselves. For an actor to play an actor requires a level of imitation that encompasses the whole of the character's on-screen and off-screen persona, and this inevitably means being measured up to and compared against the skill and integrity of the very actor being portrayed. Can anybody really capture the elegance and grace of Audrey Hepburn? Can Andy Kaufman's instinctive comic timing be replicated? Can anybody pull off an impression of not just Peter Sellers, but all of his voices and characters too? Its a hit-and-miss business which relies on more than just a physical resemblance or a matching wardrobe. The believability of a part can hang solely on the likes of genuine emotion and subtle mannerisms. If its not right, its just a caricature. In a long list of stars playing stars, some stand out far more than others  though not always for the right reasons. Here are ten of the most notable instances of Hollywoods finest being resurrected by other actors, shining a spotlight on the ones that got it right, and the ones which were shy of the mark.