De Niro stars in director Justin Zackham's movie as a divorced man who pretends he is still with his ex-wife for a family wedding.
In an interview with USA Today, Sarandon said she was especially impressed by a scene where De Niro's character comically proposes in the presence of a priest (Robin Williams).
"When you think of [De Niro], you generally think of these tough guys like Raging Bull or Taxi Driver... This was a once-in-a-lifetime opportunity. And he did the scene in a very moving, adorable way," the actress explained.
Sarandon also joked that men should look to De Niro's character as an example of how men should treat their wives.
"He plays someone not at all sensitive to women's needs, but he comes around. It shows there's hope for all men," she quipped.
Katherine Heigl, Topher Grace, Diane Keaton and Amanda Seyfried also star in The Big Wedding, opening on October 26 in the US and November 9 in the UK.