Hitchcock was very fortunate to have cast the young Anthony Perkins in the leading role. At the time there were some other young aspiring actors who might have qualified for the part, namely: Michael J. Pollard; Jack Nicholson; Bruce Dern, Dean Stockwell, Victor Buono and Dennis Hopper. However, it's hard for me to imagine anyone other than Perkins playing Norman Bates with complete authenticity, it's seems the role was simply made for him and only him. Could you possibly imagine anyone other than Nicholson in the role of Randall P. McMurphy in "One Flew Over The Cuckoo's Nest" or Dennis Hopper as Billy in "Easy Rider"? Keep in mind the introverted, insecure Mama's Boy was not really PSYCHOpathic as Hannibal The Cannibal was in "Silence of the Lambs" - a cold-blooded killer with no conscience - but rather criminally insane, plagued by schizophrenic delusions and seriously in need of multiple reality-checks. Anthony Hopkins did a wonderful job as the vicious Hannibal, and also as Corky, the schizo-killer ventriloquist in "Magic". I think he just might have pulled off a convincing Norman, but don't believe he was available back then. Oh well, so much for speculation...Perkins did a great job.
