Under what pretense does O'Brien stop to talk to Winston?

O'Brien stops Winston in a hallway. His pretence is to discuss recently obsolete words that Winston has used in his articles. O'Brien also complements Winston on his elegant writing style. It is, of course, dangerous to be too good at your job but Winston is still awe struck and flattered. O'Brien mentions the recent 10th edition of the Newspeak dictionary that Winston might want to come to look at. This sets the stage for their "revolutionary" meeting and Winston's subsequent capture.