> As long as you're willing to risk arrest and expensive lawyers, don't say anything.

Lawyers are not automatically expensive. In fact, a decent lawyer can often save you money. It does require that you have the resources to pay for the lawyer in advance, and if you cannot then you have my sympathy, but my default position is yes. I get a lawyer for everything, even simple driving violations.

In the kidnapping scenario described above, I would definitely have tried to help the responding officer by describing what I saw and where the person fled, but anything beyond a very cursory exchange and it's 'I would like to help you but I have no further comment without speaking to a lawyer first'.