Before he officially turned in his script for The Last Jedi, writer/director Rian Johnson toyed with a variety of ideas of what happened following the events of The Force Awakens. One of those ideas and its handling of Captain Phasma would
“When I was first writing, I got really excited because I’d written a line where she said, ‘I was stuck in that garbage thing for three weeks,’ and [Finn] gives her a look and says ‘What did you eat?’” the filmmaker told io9.
The exchange would definitely have been hilarious to watch unfold in The Last Jedi, but the events of The Force Awakens would have resulted in a continuity error.
When Finn and Han encounter Phasma, they force her to lower the shields on Starkiller Base before tossing her into a trash chute. This allowed the Resistance to get close enough to the planet to ensure the base's destruction, essentially demolishing the trash chute in which Phasma potentially would have lived.
“Then I realized, no, [Starkiller Base] blew up,” Johnson corrected of his initial plan. “If she was in the garbage chute, that means she blew up with the planet.”
“We needed her alive here,” Johnson pointed out. “Mostly because I wanted to hang out with Gwendoline Christie.”
From our very first glimpses of the character, fans were excited to see what made Phasma so special that she deserved to wear chrome armor, only for her to only be seen in the film
"I think they put her into the garbage shoot and it's taken a long time for that smell to wear away, and I think just, obviously there are the circumstances but the smell you see," Christie told ComicBook.com. "When you can't get rid of a smell that can absolutely plague you. That's not something I personally experienced, but it can make you volatile, and I think she's off the chain now, with Finn and the smell, she's lost control."
We'll get to see just how dangerous Phasma can be when she returns in The Last Jedi on December 15.