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As well as logistical, technical, and artistic issues, you will nearly always have to deal with some legal issues when shooting video. It is always necessary to get permission before filming people or on private land. Filming itself is not the problem, but rather public release of filmed material. Getting permission after the event is often difficult and sometimes impossible, so it is always better to deal with permissions before you start shooting.
It is not always necessary to get written permission to shoot, but where money changes hands it is usually advisable to use a written contract, if only for tax reasons.