She made the decision "because Miley [Cyrus] asked me herself to play her mom," Preston, 47, tells PEOPLE. "I was really moved. It was the perfect movie to sort of get my feet wet."
As they filmed the Nicholas Sparks drama in Tybee Island, Ga., last summer, "Julie Anne [Robinson, the director], Adam [Shankman, the producer] and Miley took such great care of me," says Preston, who brought her daughter Ella Bleu, 9, along for the trip. "They were wonderful with my schedule. I just worked a little bit."
But she spent a lot of time bonding with Cyrus off screen. "We've been friends for awhile," Cyrus told PEOPLE in January. "We got even closer on this movie so it was cool to spend the summer with people you really like."
"I love her so much," Preston says of her costar. "She's such a good girl."
The Last Song hits theaters March 31.