Images used for news or artistic works do not normally require a model release. In the first case, it would be unrealistic to expect a newspaper, for example, to get model releases before publishing pictures of a large group of people in a protest. For artistic purposes, there are a large number of street photographers taking pictures of people on the streets for artistic purposes, again without release. Now, in that case, if there was a desire for the image to be used for commercial purposes, such as stock for Getty (which is where I think you're going based on your previous question), they'd be out of luck.

So, for candid photography, getting a model release does give you some additional flexibility in the use of the image, but it's not actually required if the image isn't going to be used for commercial purposes. As for approaching them, well, that's tough... I'd have a hard time with it, I think, but not everyone does.

For actual working shoots, and I've only done a couple, the answer is sign everything before shooting. This includes any model releases, and possibly the buyer contracts, that stipulates the usage rights and other details, including payment. Never do the work until you have all those figured out, it protects you and it protects the models. Don't worry, people modeling for commercial purposes generally expect the release and they won't be shocked by the contents of it.