This is a scene near the beginning of Alfred Hitchcock's The Thirty-Nine Steps:
Here, Richard Hannay (Robert Donat) makes three attempts to convince the milkman (Frederick Piper) to help him. Hannay first tries to bribe him. This would make help a commodity that can be bought and sold. But the milkman wants to know more ("What's the big idea?"), so Hannay concludes that "I'll have to trust you." He then tells a story about a murdered spy in his apartment—which is actually what happened in the previous scene. When this truth proves unconvincing, Hannay tries again, this time preceding his story differently: "I'll tell you the truth." Even though the story of adultery that follows is pure fiction (and in fact just as cliche-ridden as and hence no more credible than the story of the murdered spy), the milkman is finally convinced to help Hannay.
When Hannay can't buy help, he has to tell a story. And given that the milkman, as the saying goes, "doesn't buy it," what Hannay has to do is sell a different story. He has to give the milkman a story he wants to hear: not "a lot of tales about murderers and foreigners," but the kind of story he likes, about adultery. As the milkman says, "I only wanted to be told"—and what Hannay learns is that what people "want to be told" is not the truth, but something that they will "buy" because it is what they want to hear.
If Hannay thus discovers the utility of an effective lie, there is still a way in which Hannay is always telling the truth: he does need to "make a getaway" from the two men who are outside in the street. His intention is true, even if the story he eventually finds to realize his intention is not. This intention remains true throughout "The 39 Steps," even as it gets complicated by a second intention: Hannay's attempt to prove that he did not commit the murder in his apartment.
In order to prove himself innocent, Hannay has to learn, as he does here, that the story of his innocence is beside the point. The truth may come out in the end, but not because he tells people the truth. Nor can he buy people's trust; rather, they have to buy him by believing his stories (true or not). In "The 39 Steps," the truth may need to be revealed, but it is best gradually revealed by the telling—and selling—of effective lies.