A plotline i didn't understand......

In order for their plan to work, it was very important that Fitch perceive them as entirely third party, out for money, not pro-gun nor anti-gun types. Being able to swing both ways allowed them to really threaten his case, thus making it worth a great deal of money to bring them on board, thus producing a blackmale against him for 10 years as well as money for their hometown. Also, if they just came to him, it could provoke suspicion, in the "are they out to get us into a scandal?" type of thing.

As far as plot goes, it's an interesting twist-creator, but you can sort of infer their true motives in some subtle ways. Just look at the way she looks at Hoffman's character when presenting a deal, then look at how they speak to Fitch. You can detect a great deal of sympathy in them toward the anti-gun people when you put the pieces together, so I don't think it was really meant to throw the viewers too much.

Plus, from a total story standpoint, the story does everything it can to establish the two as protagonists, the gun lobby as the bad guys (shady room with owners smoking cigars, arson/attempted murder at random intervals, etc), and the Hoffman character as the naive idealist good guy ("I believe in the jury system," surprised that his witness gets messed with, etc).

1) They have to make Fitch think that there just doing this for the money. If they wouldn't try to blackmail Dustin Hoffman, Fitch would become suspicious and maybe look for a political or personal reason for the jury manipulation.

2) It's the only way Fitch would actually pay. Weisz and Cusack knew Hoffman wouldn't pay and when that moment happens (the phone call that Fitch is easdropping on) he is convinced that they'll swing the verdict his way.
Also it would be suspicious if Cusack and Weisz wouldn't try to make maximum profit by playing both sides

Am I making any sense???........

lepanto, what a great observation. Although an all-time exellent movie for me, this same thing was always in the back of my mind and I had never heard the same thoughts (so well) expressed by another.

I kept rationalizing this and explaining that Nick and Marlee had been preparing for this for many years and had taken extensive martial arts training. Then the courage of their convictions helped them win out in some of the fights I guess.

But it's still weak - these were professional thugs they ran into. The strategies in the negotiations with Fitch & the jury I bought. But the fights - these should have been more believable. Nick and Marlee could have escaped through some fate/dumb luck situation in the encounters, but not by out-fighting these guys.