Winkler, who is best known for playing Fonzie on the long-running TV series "Happy Days," was on the set of Ritter's show "8 Simple Rules for Dating My Teenage Daughter" the day Ritter died from a tear in his aorta -- Sept. 11, 2003.
Winkler is expected to testify Wednesday, the Los Angeles Times reported.
Ritter's family is suing two doctors for the $67 million they say Ritter would have earned had he not died.
The family's lawyers contend the doctors could have saved Ritter's life had they diagnosed him properly and treated him differently.
But defense attorney Stephen Fraser argued the plaintiffs' case is based on "flawed science, wrong assumptions, speculation upon speculation upon speculation, all wrapped up in the obvious aura of celebrity," the Times said.