One way to put the difference is that Emotivism and Quasi-Realism differ in how a reasonable person should interpret the moral claims of others, like "stealing is wrong."
If Jill says "stealing is wrong," a reasonable person should, according to:
- an Emotivist, treat that statement as merely an expression of Jill's feelings about stealing, comparable to "booo, stealing" or "yuck, stealing." It is not a sentence with the same grammatical structure as "snow is cold," which expresses a potential fact. Instead it is linguistically more akin to a moan or a snarl.
- a Quasi-Realist, treat that statement as if it were a claim Jill is making about the objectionableness of stealing. It attributes the property of wrongness to the activity of stealing, just as Jill might attribute coldness to snow. However, the Quasi-Realist, and perhaps even Jill herself and the reasonable interpreter, realize(s) that wrongness is not a real property of things.
Emotivists like Stevenson and Gibbard and Quasi-Realists like Blackburn are somewhat more sophisticated than this, but those are the basic positions.