Guys, it is very jurisdicition and fact dependent. I cannot tell you anything that would be useful in your state, unless that state happens to be California.
So here is some California law: we have defamation per se, which means you say something nasty about someone’s professional abilities or criminal proclivities (and a couple of other things.) Under California law, you are presumed to have damages in this cases. In a non per se case, such as Bob never donates to charity, you have to prove damages. I don’t take those cases.
Now, suppose I say that Bob is a bank robber, and not a very good one at that. Well, I am liable to Bob for accusing him of being a criminal, and he need not prove damages. Moreover, I’ve accused Bob of being bad at his chosen profession, so again I am liable. In response to Bob’s suit, I say, well, what I said is true! If I can prove that Bob is a bad bank robber, I win. (I probably win if I can prove he is a good bank robber, unless the jury hates me and loves Bob, but you get the point.)
There is a whole body of federal law related to the first amendment that protects people from suits brought by public officials and figures in varying degrees, but as a practical matter, I don’t find it comes into play in my practice because I am never dealing with public figures.
Malice in non-public figure cases is proven by the context in which it is used: it can be inferred from the statement itself. I love Bob dearly, but he is a bank robber and a bad one at that is tricky. Bob may be someone I care about. What were my reasons for believing it to be true. Making the statement with an inadequate basis is grounds for malice. Each state’s common law probably specifies a number of ways to infer malice.
As for our NJ situation, you need to talk to an NJ lawyer, preferably one who specializes in defamation. It is a weird area of the law, so don’t trust a the judgment of somebody who hasn’t completed a few defamation cases. The burdens of proof are sidewise and backwards from other cases as both a practical matter and sometimes a legal one. To top it off, you have another lawsuit going on, so your side’s top lawyer needs to know what is going on and whether your expert bringing suit will help his case or hurt it. I thought this was a weird situation, but then, I have just such a case going on now with a defamed expert. In my case, it helps the case in chief.