I've had a lot of difficulties with successfully casting someone in the role of Spitfire for my project. I have a script and I have a vision, and I have the money. I just need someone with the right voice!
In The Maretian, Spitfire starts out mostly the same as she appears on the show. She was brought on partly because of her experience in running the Wonderbolts, and partially for political reasons. They already had Starlight Glimmer because she helped build the Sparkle Drive, and worked with the Equestrian Space Agency, and Cherry Berry from the Changeling Space program, the pilot. That's a unicorn and an earth pony, so Spitfire rounds it out with a pegasus on the crew. She's the team medic, as well, but past that, doesn't have much to do early on.
When the Equestrians start learning how to speak English for Mark Watney's benefit, Spitfire takes the longest to learn, and even by the time they get to Schiaparelli crater where the Mars Ascent Vehicle is, her English is still a little bit broken. (Her Equestrian is completely natural, obviously.) Most of the lines we need for The Maretian right now are of Spitfire speaking English, but there might be times in the future where she speaks in Equestrian.
In the story, Spitfire's story arc is her need to feel needed. When she starts off, she's bummed because she's gone from a command to being the oldest and lowest-ranked member of the crew, put in a role she's only marginally qualified to fulfill, and deprived of her single greatest natural talent (flight). By the end of the story she's saved the mission at least twice, grown into her duties, and established herself as equal to the others, but at the cost of losing her competitive edge (and possibly other long-term health issues, hinted but not addressed in-story).
(Incidentally, the majority of the reason why she continually rode Starlight on health issues was so she could feel like she was doing something positive. Patient = purpose.) (and her feeling of being the least qualified member of the crew is why she was the most sensitive on the subject of being talked down to / infantilized by Mark.)
For Equestrian, just act as Spitfire normally (in real-world English), but for English from Spitfire, you have to act like it's a foreign language to you and you're struggling to form words. Reading English is also difficult for Spitfire.
[lines from the series, to test voice match] [from "The Best Night Ever", S1E26] Always hungry after a show. Eh, Soarin? [to Rainbow Dash] Hey! I know you. You're the pony that saved us in Cloudsdale and won The Best Flyer Competition. Well, Rainbow Dash. Looks like your skills saved us again. Oh, well, at least they saved Soarin's apple pie. Wanna come hang out with us? [from "Hurricane Fluttershy", S2E22] Nice job, Rainbow Dash. You may not have set a new record, but you showed a lot of guts.
[a line from the story written by Kris Overstreet] [reacting to the way Starlight Glimmer is messing with them in their games of Dungeons & Dragons (yes, they're with Mark Watney on Mars in the Ares III Hab) each night.] But it so stupid! Go to pirate town, there Rainbow... I mean Mo-No-Chrome. Stupid name. Go to sea-ponies, there Pinkie. Go to griffons, there Rarity. Go to big city, give up quest, get regular job, and Zoe the Great and Powerful, not Tricky - oh no of course not, shows up and burns city down. No escape!!
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