Ha, ha, yes.
He did something similar in a BBC show, years ago, when he and Peter Jackson were interviewed.
They spoke normally for some time, until Jackson said something more or less remarkable. Andy reacted "He'ss the Massster", or so, in a perfect Gollem voice. Hilarious effect, massive...
True. It's on the film.
It is "projected" (replaces his image) by CGI in the frames of the movie they made of him.
He himself compared it to wearing an ape-suit and having an ape-face make-up (which we would now consider ridiculous) and described it as getting make-up and being dressed after...
In the case of Gollum (and Cesar in The Planet of the Apes) Andy Serkis played the part - and then after the takes, his motions, his voice, his facial expressions, were replicated on CGI versions of Gollum and Cesar (who don't exist, of course). In a way it's still totally him, but it just no...
Yes and no.
The CGI version is "projected" on Andy Serkis playing the part (and delivering the voice - actually the voices) while wearing a sort of surface 2D wire suit.
He once described it as getting the make-up after, instead of before playing the part on location (or stage).
Cees