While some will have you believe that the idea of "stunt casting" is in some way a dirty concept, it's no mere coincident that some of the most impressive movie performances of all time have come from unexpected places. You don't remember Sylvester Stallone for his action movies as much as for his quiet, human performance in Cop Land. You might buy more tickets to Adam Sandler comedies, but it's his dramatic roles that really leave a mark.
That's why there's such a currency in making seemingly strange casting choices or encouraging actors to really let loose and embrace the extremes of their characters. It pays off particularly well when the characters are dark, because the darkest characters are invariably the most interesting and the most lucrative for performers.
And over the years, there have been some incredible, disarming performances by actors going to the dark side...
11. Robin Williams - One Hour Photo
There's a tendency to believe that Robin Williams' film career was solely comedic, given his background as a stand-up, but after getting his start in an ACTUAL porn comedy anthology Can I Do It 'Till I Need Glasses? and starring in the disastrous Popeye, it wasn't until he went more serious in 1987 with Good Morning, Vietnam that he really broke out.
And yes, he went far more comic in the wake of Aladdin, but he'd made The Fisher King, Dead Poet's Society and Awakenings before then and had obvious dramatic chops. That still prepared nobody for how stunning he was as lonely psychopath Seymour Parrish in One Hour Photo.
Chillingly one-tone (which was most disarming because of Williams' usual vocal flamboyance), Sy was a picture of slow-burning, not-quite-right creepiness; all restrained rage and violence and Williams played him to haunting perfection.