If you receive money your status is different. I.E. an honoraium from a university for a lecture series you are Business not Tourist. I.E. begging for a song may result in a citizen receiving a citation - a tourist deportation. What is cute in Picadilly is a crime in Portland.

CONSULT: http://www.msri.org/local/visa/

If you are to receive local and/or travel expenses, it is again critical that you inform the official and obtain the "WB" designation on your I-94 card.