Long time BTC dealer here from Rio de Janeiro, one of the most insecure places in the world.
We always have a really paranoid protocol ourselves (three people, the dealer don’t hold any cash and meet with a burn phone with no wallets or passwords to access anything, yadda yadda), but it depends on whether you are buying or selling and the amount.
If at all possible, have someone with you and agree on a subtle sign to warn if there is anything wrong, specially for new customers.
If you’re RECEIVING cash, check for counterfeit bills (may be really hard). If we deal high values, we do it inside a bank and only release the coins after we deposit the money so the bank machines check the bills. If it’s a small value, you can get a “counterfeit money checked pen” of sorts for cheap and check some of the higher bills randomly. Also, going away later on can be dangerous because the person will know you are carrying the money, so be aware and try to get somewhere safe ASAP.
If you are GIVING cash there is less to worry, but be careful while approaching the person and make sure the coins will be released/sent (and conrfirmed!) before the person goes away with the money.