Katherine Heigl has signed to star in the indie comedy "Gently Down the Stream" alongside Robert De Niro, Diane Keaton, Susan Sarandon, Amanda Seyfried and Topher Grace. The movie, which will be retitled as The Big Wedding in international markets will be written and directed by Justin Zackham ("The Bucket List") and is a co-production between the filmmaker's Two Ton Films banner and Millennium Films.

Zackham and his Two Ton Films partner Clay Pecorin will produce with Richard Salvatore and Harry Ufland, while Avi Lerner, Danny Dimbort and Trevor Short will exec produce.

Lerner's Nu Image/Millennium Films will be selling foreign rights to the film and have promoted the picture at the Cannes Film Festival, where they have already closed a distribution deal with Telemunchen / Concorde in Germany.

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The film is about a dysfunctional, divorced couple who pretend to be still married at their adopted son’s wedding in order to please the guest of honor: his religious birth mother. De Niro and Keaton play the couple who quickly learn that acting the part isn't easy, especially when their past, present and future unravel for all to see.

Topher Grace recently told Slate Magazine about his role as the son of De Niro and Keaton's characters and explained that he and Katherine had been talking about the production which is scheduled to start this July in Greenwich, Connecticut.

"I play their son, and Katherine Heigl plays my sister, and Susan Sarandon is our stepmom. I just hope they all come ready to learn. I've got a lot to teach! ...No I'm just kidding. I'm excited to film with Katherine, who I knew from when I was younger because we both grew up in Connecticut. We're actually shooting it in Connecticut, so we were talking the other day about how excited we were about that."

Speaking to Variety, director Justin Zackham explained, "I'm excited to be able to continue what we started with on The Bucket List - iconic actors in a funny, adult, character-driven story, albeit with a little more sex and bad behavior on everyone's part. Nu Image/Millennium has been a great partner and I'm honored to have such an amazing cast,"

"Working with Diane and De Niro again will be great," said Avi Lerner. "Katherine and Amanda are young talented actors. It is good casting. Justin's script is well written and he is the right choice to direct. It is going to be warm-hearted funny film," he added.

Robert De Niro and Diane Keaton have worked together twice before, on "The Godfather Part II" and "Marvin's Room." It will also be a second film for De Niro and Heigl, having recently completed work on the forthcoming ensemble comedy New Year's Eve, scheduled to be released on December 9th.