This kind of part is perfect for Keanu. Dim-witted, slow and unsure of his surroundings.
Richard Linklater (the director) did another film like this... Waking Life I think it was called. At any rate the rotoscoping certainly augments the drug induced paranoia and halucinations.
I know that the the first part would be... interesting if watched under the influence.
According to at least one reviewer, the story suffers form the book to film transition. Mainly beacause the book has many internal monologues which help convey a deep sense of paranoia.