It’s the kind of criticism that really cuts to the quick because Guinness seems to hate everything that Daniels did with this famous role. He didn’t simply dislike a single scene or a particular acting choice. C-3PO being fidgety and a prissy chatter is basically his whole character, meaning that the late, great Guinness hated this performance from top to bottom.
It’s worth noting that the Obi-Wan Kenobi actor may have had some other resentments coloring his assessment of the C-3PO actor’s performance. Right after his harsh criticism of Daniels’ performance, Guinness said with some apparent bitterness “Not that any of the cast can stand up to the mechanical things around them.” While modern fans hold up the Original Trilogy as a golden age before the dark times of endless Star Wars CGI, it’s interesting that Guinness felt even back then that the human performances were getting overshadowed by the artificial ephemera Lucas filled almost every scene with.