I am thinking about that in relation to Oswald in regard to two things he had to be totally unprepared to lie about: the trip to Mexico City and the Backyard photos.
There is no reason to think that Oswald was prepared to lie about Mexico City. There is no reason to think that Mexico City was even remotely on his mind. So when, from out of the blue, the FBI agent asked him if he recently went to Mexico City, Oswald would not have had a lie prepared, and before attempting to lie, he probably would have paused and thought to himself, "How does he know about that? Why is he interested? What should I do? Should I lie, or should I tell the truth?" And the very pause, the very equivocation, would have destroyed any denial that followed. In other words, there was no time for Oswald to think. There it was, right in front of him, on the spot: "Did you recently go to Mexico City?"
I would argue that since no one has ever accused Oswald of committing any crimes in Mexico City that his mind would never have calculated a decision to lie about it. Again, there wasn't even time to make the calculation to lie. And, the default is not to lie. The automatic, normal, usual thing is to answer truthfully. Oswald would have answered truthfully about Mexico City, and he did answer truthfully, that he had never been there, and the only place in Mexico he ever went was Tijuana. It was a natural, spontaneous, undevious utterance.
And then in regard to the Backyard photos, they were allegedly taken in April, shortly before Oswald left for New Orleans, although when first asked, Marina said they were taken in February. But, so much had happened since then, it's unlikely that those photos were on Oswald's mind or that he was prepared to lie about them. After all, he, presumably, asked Marina to take them. He was the director, not her- right? So, why did he want the pictures? To show them to people, right? Or, are we supposed to be believe that he wanted those pictures just to look at them himself? So, there is no reason to think Oswald was prepared to lie about them.
So, they pull this photo of him out, and immediately, he recognizes it and know it's him, but yet, he bold-faced lies??? He' all ready to go with a lie? It just rolls off his tongue? "Oh, that's another guy with my face pasted on." On the spot, he came up with that????
No way. He must have realized immediately that it was NOT him, that it couldn't be him, and that's what caused him to study it so closely. And, he said it was a forgery because, indeed, it looks like one, with the face plastered on. He didn't just pull that from out his ass.
The point is that Oswald didn't lie about these things, and he wasn't even prepared to lie about them.