The two stars are locked in a legal battle in Louisiana after Baldwin accused Costner and his associates at Ocean Therapy Solutions of locking him out of a deal that would have made him a multi-millionaire.
Baldwin and his business partner Spyridon Contogouris claim The Bodyguard star cheated them by hiding details about a deal made with BP following the Gulf Coast oil spill in 2010, and allege they should have been told about the potential agreement before they agreed to sell their shares in the company.
The Usual Suspects star testified in a New Orleans court on Monday (11Jun12) and told the jury that he should be compensated for lending his name to the marketing of the oil extraction devices invented and created by Costner's brother.
He said, "When you're famous, it opens doors and things like that."
Costner attacked Baldwin when he took the stand last Friday (08Jun12) and insisted he often "wondered" what the born-again Christian was doing behind the scenes.
But Alec Baldwin's younger brother is adamant his only job was to "market and promote", adding that he had "no responsibilities relevant to getting the devices in the water".
He continued, "I wanted the machines to work. Make some money? Great. Not my first motivation."
The plaintiff also insisted he would have been able to help raise funds to invest if Costner and his partners had asked, even though he faced his own financial troubles.
BP bosses made an $18 million (£11 million) advance payment for 32 devices developed by Ocean Therapy Solutions in an effort to be better prepared for another oil leak disaster. Baldwin and his business partner are demanding damages of at least $15 million (£9.3 million).
The case continues.