Why Does Mantegna Come To Restaurant?

One of the many things I appreciate about this film is the amount of room Mamet leaves for interpretation. Until Mike brings up the watch, everything was relatively above board. Mantegna informs Mike that Chet wants to do a deal with him and he should get himself a lawyer. If his intent was to steal the idea, he would not go on record with another face to face and suggest hiring counsel. It's safe to assume that if Mike doesn't mention the stolen watch, they would cut Mike in on the reality show angle and perhaps the co-producing/consultancy gig on the war film. Instead Mike mentions the watch, the stolen watch which Mantegna purchased at Chet's behest, and the jig is up.

Mantenga is now exposed and needs to cut Mike loose to protect his own interests and relationship with Chet. Mantenga, not wishing to explain to Chet how he got him mixed up with a stolen watch, decides its easier to screw Mike and heist his ideas. Film people are very, very insular with their relationships and while Chet likes Mike he is still an outsider. Mantenga is a trusted friend and co-worker who simply tells Chet whatever negative yarn he likes and the star & his wife subsequently distance themselves from Mike.

Joe meets Mike for dinner because Mike is just another asset to him to make Chet more money and by in turn increasing Joe's salary/worth. Joe is in no way finished with Mike, he see's mike as a wealth of information and experience that he could work with to make money. You already saw how slick he was at gleaming info off Mike about his military experience and the next day he was in a military film set which then led to him getting the script and making it more authentic etc. Thats what Agents do, they acquire contacts and set them up, making things happen for their client to earn the bucks.

Whether or not Mike is down with the making money plan is another matter, when he mentions the watch, Joe realises that this is a threat to Joe and Chet's relationship so he decides to cut Mike lose but tries to at least get something out of it by shopping Mike's ideas around to see if he can earn money off of it.

It was all a con to force Mike into fighting. The magician, Joe Mantegna, Tim Allen, the promoter, his wife and her family were all in on it.

To the OP,

You obviously don't understand that they were stringing Mike along, because they weren't. One thing many fail to understand is that dealings between Mike and the movie were on the level. It wasn't until Mike confronted Montegna about the stolen watch that he decided to screw Mike over. Them even finding out about Mike's idea was pure chance.

Dig, Montegna leaves dinner and tells Allen and his wife that they're demanding more money and advises them that they don't need his idea for the movie (which wouldn't be hard to believe). Montegna then sold the idea to Ricky Jay barely even giving a thought about Mike. Mike would be hard pressed to win a court battle with Jay until he found out that Montegna and Jay were in business together.