The biggest challenge in photographing people is to capture candid moments without intruding on people's space. Getting buy-in from your subjects requires a mix of attributes unique to every successful people photographer. I learned a lot from watching photographers like Steve McCurry work. He doesn't call attention to himself with flashy equipment or weird clothing.
He blends in, smiles a lot, works with a simple bag slung over his shoulder, and uses his charisma to engage with subjects as he works. Perhaps the best and most valuable ingredient, however, is time. Having the time to get people comfortable with your presence, spending time in communities, and working with leadership so that people understand why you are there. The worse mistake you can make is being disrespectful or appearing to have shady intentions. Nothing will get a subject to relax more than knowing that you are there to tell an honest story.