7. Molly Brown - Margo Martindale
While the Titanic movie has a grounding in reality, it's not news to reveal that most of the events on board were fabricated for dramatic tension. There was no Rose, no Jack, and most of the characters who did exist probably didn't behave the way you've seen in the film, save for maybe Molly Brown.
Known as "The Unsinkable Molly Brown", this figure was one of the biggest heroes of the tragedy, attempting (unsuccessfully) to get the lifeboats turned around and even leading a survivors committee to help lower class passengers after the disaster.
The character needs someone who can convey strength and compassion in a short amount of time, and legendary character actress Margo Martindale would be an ideal candidate. She can fit into any part she's given, going from dramatic and profound roles in projects like American Crime Story to hilarious and self-aware bits in the likes of BoJack Horseman.
Martindale would also nail Molly's unconventional placement in the upper class, which helps her bond with Jack and show support for the lower classes. She would, no doubt, do Molly Brown proud.