 The effects team spent six months creating the dementors.  Originally, Alfonso Cuarón (qv) wasnt thrilled with the idea of  using CGI and wanted to use more traditional techniques such as  puppetry. The filmmakers tried a basic technique with a dementor  puppet floating in the breeze but werent satisfied with the way it  looked. Puppeteer Basil Twist (qv) showed them a technique that  involved putting the puppet in water and shooting it in slow motion  with the film reversed. The filmmakers liked the way it looked but  realized that this would be an impossible task, so in the end they  decided to use CGI to create the dementors.