This paper aims to analyze the design aspects responsible for the believability of computer generated characters in live-action movies. This research proposes the hypothesis that synthetic thespians constitute an ontologically distinct category of characters - to which the term Digital Actor is proposed - based upon a design process that involves three development contexts: the diegetic, the perceptual and the technological levels. In order to carry out such investigation, the case study methodology is used, in which the creature Gollum, from the movie trilogy The Lord of the Rings (2001-2003) is analyzed. The intrinsic variables investigated in this case show that Digital Actors in fact suggest an original design approach and therefore a new role for the designer.