[Michele]

Acting can be dangerous but it might depend how dangerous exactly. Telling a lie that would clearly result in the hostage-takers being captured/killed is very dangerous. Feigning sympathy for the criminal, OTOH, is pretty standard, and if the negotiator gets caught at being insincere in that, it might irk hot-tempered criminals but not colder types.

I would add Body Language. Often, negotiators have to rely on spoken input only, but if they have visuals that may be of great help.
Savoir-Faire (criminal, maybe with an optional specialization in, say Somali pirates or something) may be useful if this is not an individual taking hostages in an impromptu situation. If these are professionals, there may be conventions about the dealings with them.
Again thinking about them Somali pirates, negotiators who are experienced with ransom kidnappers also have high scores in Merchant; it's part of the reason why the insurance companies go through them.