Not now. Not now. When it's all over. When it's behind us—then—.

Mary Debenham to Colonel Arbuthnot speaks this quote, found in Chapter 1, Part 1. On the Taurus Express to Stamboul, Poirot, Mary and Arbuthnot are the only passengers aboard the train. Poirot gives the couple, acting as strangers, close scrutiny. He hears Mary say this quote to Arbuthnot when the train is stopped at a station.

This quote is a very important clue. For one, the apparent close, intimate relationship between Mary and Colonel Arbuthnot is suspicious because the two act like they are strangers. It is unclear what event Mary is talking about, what will be "over." With the unfolding of events on the Orient Express, Poirot immediately assumes the event was Ratchett's murder. Poirot's suspicions are heightened because Mary and Colonel Arbuthnot refuse to tell Poirot what they were talking about.