Check out some great tips on shooting your film without causing your film shoot to be canceled. Bottom line, plan everything out on paper first.
http://www.indiewire.com/article/shooting-guerilla-style-at-your-own-risk-the-8-tips-you-need-to-know
http://www.indiewire.com/article/shooting-guerilla-style-at-your-own-risk-the-8-tips-you-need-to-know