When Jennifer Aniston’s character Rachel Green on “Friends” needed someone to play her mom, the actress suggested Marlo Thomas. “Her father, John Aniston, and I had done a play together before Jennifer was born,” Thomas recalls. “We played brother and sister. When I got the call to play Jennifer’s mother, I said, ‘That’s funny — I already played her aunt!’ We had a great time. She was very excited to have me there, because she’d remembered gifts I’d sent her and her brother when she was small.”

Their rekindled friendship led Thomas to ask Aniston to start appearing in public service announcements for St. Jude Children’s Research Hospital, the pediatric facility in Memphis, Tenn., that provides free medical care to children with cancer and other life-threatening diseases. The doctors and nurses at St. Jude treat 8,000 patients a year from all 50 states, with housing available to them and their families at no charge. Since 2006, Aniston has filmed an annual PSA for St. Jude, sitting next to the young patients as she talks with them and asks the public to donate money to the nonprofit.

Peggy Sirota for Variety

“I started doing everything I could, meeting the children, and they’re these little heroes,” Aniston says. “I remember one year the little girl kept calling it her ‘owies,’ her little tumors, because they wouldn’t say the word ‘cancer’ in front of her. It’s the hardest day of the year and the greatest day of the year, because they allow you to put it all into perspective. You see these little miracles, these heroes, walking through something that no child should ever have to be walking through.”

Thomas — whose father, the actor, singer and philanthropist Danny Thomas, founded St. Jude in 1962 — chose Aniston to be part of the campaign because she knew she’d have a strong bond with the children and the view­ers at home. “I was in the medicine room of St. Jude one day, and a mother said, ‘Will you give this note to Jennifer Aniston? She saved my daughter’s life.’ How is that?” The mother went on to explain: “I didn’t know what was the matter with my daughter who had headaches. I saw the commercial, and Jennifer was talking about this little girl, and I realized those were the symptoms. I went to the doctor and said, ‘Jennifer Aniston told me these symptoms. I want an MRI.’ And in fact, my daughter did have a brain tumor.”

“I stay in contact with the families sometimes,” Aniston says. “Most of these kids are from places that don’t have the opportunities, and necessarily income, that some families have the good fortune of having. And it’s a beautiful environment. Every year, I get this birthday card that St. Jude makes with all their faces on it. And each of them signs it. They are treated like the angels that they are.”