Here is a pretty through answer.
What youre talking about would be a civil matter, not criminal, because it's dealing with the rights of private citizens. So if someone was to take this up with you legally, you wouldnt be criminally prosecuted. They would take you to civil court to either sue you or make you take the video down.
Keep in mind they would have to hire a lawyer, which would be expensive. Before taking you to court, their lawyer will send you a cease and desist letter., asking you take down the content, and threatening to take legal action if you don't comply. If you comply, you'll be off the hook.
Now if you were to film a fight it could be deemed newsworthy, especially if it was in public. If it was considered newsworthy, you would not need consent forms or to blur faces.
Newsworthy is defined as - Of sufficient interest or importance to the public to warrant reporting in the media.
I wouldnt worry about it man. Even if someone was to hire a lawyer and go through all this shit, and you content wasnt considered newsworthy (which is unlikely), you would still, at worst, only get a warning. Remember, what you are doing isnt criminal.
This is what a cease and desist letter will look like -
http://www.utsystem.edu/ogc/intellectualpr...tract/cease.htm