"Sandy was the picture-perfect professional while filming The Blind Side," Bullock's costar Quinton Aaron, who played football player Michael Oher, tells PEOPLE exclusively. "I learned so much from working with her. I don't think she was aware of any alleged marital problems and always spoke highly of her relationship with Jesse. My heart goes out to her in this trying time, and I know she will overcome."
Others who worked on the set also said she seemed friendly and positive. "She was really focused on the part," says one source. "It was a hard part, and it took all of her attention. She was always very sweet to everyone. But that is Sandra: always kind, always professional."
Adds another source: "She was upbeat and working, and I didn't notice anything that was odd."
Proprietors of local eateries and retail shops in the Atlanta area, where the movie was filmed, echoed those sentiments.
When she visited the Two Urban Licks restaurant in May, "she looked great and was in an awesome mood," says general manager Chakib Touhami. "She was just cracking jokes. When I brought her out dessert, she said 'You're gonna get me fat!' She was so normal."
That month, she also visited Murphy's Restaurant and The Shed at Glenwood. "She was in a good mood, very friendly," says a Murphy's staffer. "She was super complimentary to the chef and very nice to the waitstaff. She really came across as a genuinely nice person."
Karen Briggs, owner of Green Pomegranate, an eco-friendly children's boutique, also welcomed Bullock to her store. "I've been a stepmom before, and I was really struck by her concern and adoration for her stepdaughter," she says. "She seemed to be quite excited about the potential of dressing her stepdaughter, and finding clothes for her."
Reporting by SARA HAMMEL, OLIVER JONES and TIFFANY MCGEE