As for why being the petitioner is important, in Hollywood it matters who left who.
Olivier's asking for joint custody of the couple's 2-year-old son, Maceo. As we reported, they have a prenup so money will not be an issue.
We're told Olivier and Halle have not been getting along and last week was a breaking point, after an LAX employee sued Olivier for pushing a car seat into his chest. He also named Halle and we're told she went nuclear on Olivier saying he's responsible for her bad publicity.
We're told Halle and Olivier had a fundamental personality clash. She is "very difficult" and "goes nuts when she doesn't get her way," and Olivier has a temper, though not violent toward her. It was a bad combo.