What do banks fear? Long lines! It usually signals financial instability. This drives more people to line up if they think their deposited money is in danger (FDIC insures their deposit, but not everyone knows this), which can lead to a bank run, which can shut backs down. Banks have invested money into research to keep lines short.
To hit them where it hurts, protestors can form long lines into the banks. For it to be effective, people can't acknowledge it's a protest until after it's over. In the meantime, everyone acts like a normal potential customer, asking about loans or deposits and other such services and products. This also makes it harder for banks to distinguish between protestors and legitimate customers.
I call this a line-in.