Model Releases Required?

[Mardon]

I'd like to have a better understanding of when model releases are
required. I realize that the easy answer is probably "If in doubt, get
one." but I'd like something more descriptive than that. I'd also like to
know if model releases apply to things like buildings. I know that these
answers may vary from country. I live in Canada but would be interested in
hearing how this works in other places too. My 'typical' situation is
this; I'm walking around doing landscapes when I come upon a person who
has real 'character' that would make a great photo. Is taking a 'candid'
photo of them ethical and do I need a model release for the candid photo
afterwards? Is it more appropriate to ask for permission in advance." If
I do so and verbal permission is given, do I still need a signed model
release? I also realize that the answers may depend upon what I intend to
do with the photos. The three possible ways I may use them: 1) I may
restrict showing them to family and friends, 2) I may post them on the
Internet or publish them otherwise but not receive any fee, or 3) I may
sell them. Do model release requirements differ for these three
situations? TIA Mardon