Question: At Jack Jr's birthday party, you don't see Shelby's head at all... It was her 'body' because later that day she is wearing the same shirt when she gets her hair cut. Was there a sit in for Julia Roberts in that scene?
Answer: I believe it is because the child playing Jack Jr. was basically crying through this whole scene, as you can see. It's most likely his real mother holding him to calm him down. Also, when the family is saying "Blow, blow," you can hear a female voice that is neither Julia Roberts or Sally Field.
Question: In the scene where Shelby and Jackson are eating the wedding cake, what was the point of Nancy Beth telling Shelby that Jackson is "one big-hanging man"? And how does Nancy Beth even know?
Answer: Nancy Beth, who has had a crush on Jackson, is likely jealous that he is marrying Shelby rather than her. She is trying to ruin Shelby's happiness with her spiteful comment, hinting that she has been intimate with Jackson, though it is uncertain if she ever has been.
Question: When Shelby is telling her mom about the pregnancy, what does Shelby mean when she says, "I think it would help things a lot"?
Answer: She means that she and Jackson (her husband) are having marital problems. Jackson wants a son, and she's hoping that having one will bring them closer together.