I noticed in Persepolis (2007) directed by Marjane Satrapi and Vincent Paronnaud, that cigarettes were used to signal the beginning of a flashback. The inital flashback started right after Marjane light a cigarette in the airport, the man that was in prison (I believe it was her uncle or father’s friend) light a cigarette right before he told his story, when Marjane resumed her flashback, and at the end of the film a man is smoking a cigarette as the film flashes forward to present time as Marjane leaves the airport in a taxi. I liked this subtle signal because to me it adds a sense of pain into whats about to be told. It is as if the characters are saying that they need to smoke to be able to open up and tell their tale. It does also provide a cool tansitional effect as the smoke whispers curl and fade as the shot fades to the flashback.