At the beginning, when Willard is being assigned the mission by his superiors, they ask him about missions that he had supposedly carried out in the past, but he says he doesn't want to talk about that (can he actually deny such a thing, by the way?).
What's that about?
He was tested if he can keep a secret...
shareThe scene was partly to establish to the audience that he had already been sent on clandestine missions in the past, in terms of providing some background for him, but also that he was something of an insubordinate individual. Either they already knew about his previous missions, in the which case, why were they wasting time asking him about them, or they didn't, so were they cleared to know anything at all? He was operating his own 'need to know' policy. He also didn't seem concerned about any disciplinary repercussions, because he knew they wanted him specifically for this operation, and, even if they pulled him off it, I had the impression that he wouldn't have been all that bothered by that either, because he had had enough by then and wanted out.
shareWowsers, what an interpretation. If he was insubordinate with a discipline problem, do you really think they would send him on one of the most sensitive, important missions possible?
Willard's past missions were secret. He was ordered to not talk about them, and to deny their existence when probed. That sort of order sounds easy to follow... but when face to face with a General and Colonels, refusing to answer direct questions takes on a different intensity.
The officers wanted to know if Willard could be trusted, if he was the right man for the job. This mission, embarrassing to the military and of major potential international incidence, could never, ever be allowed out in the open. They needed someone who a)can do the job, and b) will keep his mouth securely shut about it.
Even something as subtle as offering him a cigarette is a test.