Anyone on those front steps who will have noticed Mr. Oswald will not just have noticed his presence on those steps. They will also have witnessed him engaging in the behavior of
1. Greeting Pres. Kennedy with an energetic waving of at least one flag
2. Leaving the steps, taking out a large banner from a long paper bag, and laying out the banner on the ground (or draping it over the roadworks barrier?)
If they have seen (or heard tell of) pro-Castro sentiment inscribed on that banner, then they---------in their ignorance of the false-flag operation for which the faux-leftist Mr. Oswald was actually working--------will know that this man was a member of a Marxist conspiracy that killed Pres. Kennedy. They will also know that their Govt. does NOT want this fact publicly known.
The motivation to keep schtum will be overwhelming. And the motivation to expose themselves to a world of pain by coming to the reputational rescue of a man whom they believe was party to the despicable act of assassinating Pres. Kennedy will be very low indeed.
Thankfully, however, the visual record allows us to reconstruct what happened.