Waiting for the right moment is key... like when a vendor shows a child how to select ripe fruit without noticing you're there. I've found positioning myself at a distance with a longer lens preserves the scene's integrity. Another trick is to visit the same spot repeatedly until vendors become accustomed to your presence. That's when you get those unguarded lessons, like the tomato picking you mentioned. It turns documentary work into a subtle art rather than a staged performance.