I've just witnessed a bike theft, and in hindsight I handled it very badly.
(What exactly happened: I met a friend on the street, we were talking, then he noticed a pair of *ahem* gentlemen manipulating a bike (cargo/family, rather expensive) that was parked there. My friend spoke up to them, but the thieves just dismissed his remark ("somebody who has the money for such a bike can tolerate it") and carried on, and neither he nor me dared to step closer or stop them in another way. Another bystander had already called the police, but then the thieves made away with the bike before the police arrived.)
Now I feel a bit gutted for having not done anything to stop them. I haven't been in this situation before, but I have had a couple of bikes stolen myself and know well how annoying it is, how this sort of thing gets people to drive cars instead of bikes, etc..
So I'd really like to prepare myself mentally to be more helpful if I should again come in such a situation, but I don't know what one should do to make it difficult for the thieves to pull something like this off without putting oneself in danger.
Are there any established recommendations for this?