While I know that in common law countries there's no legal requirement to provide assistance, is there any liability for failing to do so once you've volunteered to?
I'm interested both in a general answer, and in a hypothetical variation of the following specific example: http://www.lfpress.com/2016/02/17/video-boy-dangles-from-whistler-bc-chairlift
A boy was dangling from a chairlift. The attendant relied on guests to hold the firemen's net stretched in order to catch the boy when he fell.
Say I'm in that situation, one of these guests, and I'm holding up my end of the net, but change my mind and let it go. What is the threshold where I would be committing a crime or opening up myself to civil liability? If I let go after the boy already starts falling? What if I let go before he falls, but while still knowing there is no time to find someone to replace me?
I'm interested mainly in North American common law context (US and Canada except Quebec).