Barack and Michelle Obama were on stage Saturday for a surprise appearance at the Martha’s Vineyard African American Film Festival to discuss Descendant, the festival’s opening night film which the former first couple helped produce.

The crowd immediately rose for a boisterous standing ovation when executive producer Ahmir “Questlove” Thompson brought out the Obamas. The auditorium was aglow from cell phone cameras snapping photos and videos.

“We are proud and honored to be here for a number of different reasons,” Ms. Obama said.

One of the couple’s post-presidency ventures has been to start a production company called Higher Ground. Higher Ground is one of the backers for a new documentary called Descendant, which was shown in front of a packed crowd at the Performing Arts Center on Friday evening.

“When we left the White House, Michelle and I talked about the things we want to do,” Mr. Obama said. “One of the things that we learned both as we were campaigning for office and taking office was the importance of stories and who tells stories.”

Directed by Margaret Brown, Descendant tracks the story of a group of slaves brought to Alabama aboard the Clotilda, the last known ship to bring captured Africans to America in 1860. After emancipation, some of the former slaves banded together to create Africatown, Ala.--a town where many of the descendants still live today.

When the couple originally saw the film they knew they wanted to get involved, Ms. Obama said.

“Our team saw this at [the Sundance Film Festival] and then we looked at it and immediately thought, this is why we’re doing Higher Ground,” Ms. Obama said.

Mr. Obama expressed gratitude to Stephanie and Floyd Rance, the organizers of the festival, for putting on an event which highlights stories that often get pushed to the periphery.

“It’s one of the powers of this festival. The work that the Rances have done is to lift up stories that have too often been lost in the flow of time,” Mr. Obama said. “We believe that everybody’s stories matter. Everybody’s got a sacred story that motivates us, moves us.”

Two of the descendants, Joycelyn Davis and Veda Tunstall, were present for the screening. Mr. Thompson also recently discovered that he is a Clotilda descendant as well. Their presence is a reminder of the visceral impact of history on the present, Mr. Obama said.

“The descendants who are with us here tonight, including Ahmir, are living proof that the past is not past,” Mr. Obama said. “It’s not some abstract idea you just read about in textbooks. It is something that shapes us every day. It is burned into us like our DNA.”