He just starred in the mini-series Hatfields & McCoys, the highest-rated entertainment program in cable TV history.
Jury selection has begun in a bitter lawsuit that pits Costner against fellow actor Stephen Baldwin, Alec Baldwin's younger brother.
Stephen Baldwin says he was an investor in the company, Ocean Therapy Solutions, that Costner told a Congressional committee in 2010 had developed a machine that could clean up the massive BP oil spill in the Gulf of Mexico.
Stephen Baldwin claims he was duped into selling his shares right before Costner landed a deal with BP to buy the clean up machines. Now, Baldwin wants $21 million in damages.
Costner is countersuing, saying he had no idea Baldwin was selling his shares in the first place and he's "surprised and offended" by the lawsuit.
Attorney Royal Oakes told INSIDE EDITION, "This is like a real life Hatfields & McCoys feud, but instead of backwoods folks duelling it out with bullets and rifles, these are Hollywood superstars fighting with lawyers and legal briefs."