Look at the probability of the outcome, given the speech - if it's high enough that you can ignore the receiver of the message as an independent agent whose response generates uncertainty, the causation looks pretty direct.

Example 1: shouting "Fire!" in a crowded theatre. Are you quite sure the crowd will stampede? or they'll look at you like you're an idiot and tell you to shut up?

Example 2: Russel and Hugh are two best mates living in Australia. Russel says: "An election is coming and I have to vote. I don't care about them slimy politicians and I'll vote for whoever you tell me". Hugh says "Sure, mate, this time vote for the Wombat!". Is Hugh's speech protected?