Whether or not you need a release depends on what you plan to do with the photo after you take it. If these photos are just for your personal use (and that includes sharing them with friends, posting them online, etc.) then you don't need a release.
If you plan to SELL the photos (e.g. you want to sell them in a gallery, or you want to post them on a stock-photo site for someone to buy who may, in turn, use them in an advertisement, etc.) THAT'S when you get to the point where a release may be necessary.
Even then... the laws can vary and it often depends on who you photographed, whether they are "recognizable" and ultimately even "why" the photo is interesting (is the photo of the person in the park playing frisbee with their dog interesting because the action of playing frisbee in the park made the photograph interesting... or is it interesting because it was a famous notable person who happened to be playing frisbee with their dog?) If the photo would have been as interesting regardless of who was in it, then in many jurisdictions you still may not need a release even if you plan to use the image to engage in some form of commerce.
The reality is, however, that even if the law is on your side (you're legally entitled to use the image the way you intended to use it without obtaining a release) that doesn't necessarily mean the other party knows the law. They may still hound you, try to extract payment from you (sue you in court) for which you'd then need to hire an attorney and basically you end up paying money to defend yourself even though you didn't break any laws. That's why some photographers (and most all stock photo sites) want releases whether they are legally required to have them or not.
But if you're not going to engage in any form of commerce... click away!
I was at an ice sculpting event a couple of years ago and, out in the area of the park where the ice-sculpting was taking place, they had signs posted everywhere that indicating that you were entering an area where many photographers were working and by entering that area you were agreeing to allow any photos in which you may appear to be used for commercial purposes.