I really like their expressions - Spitfire's bewilderment, and Dashie's restrained but righteous anger. And the way that the light's flooding in through the half-closed blinds into an otherwise relatively dark room is great, too, and creates a lot of atmosphere: it gives the picture a hot, brooding feel, which in turn fits right in with what's going on.
Spitfire was voiced by Maryke Hendrikse in season 1 (episodes 16 and 26), and either Maryke or Ashleigh Ball in season 2 (episode 22). In this episode, she's voiced by Kelly Metzger, sounding a bit higher.
secondly about attidude
The idea that she's normally a really laid back pony who has to put on the drill sergeant mask to deal with recruits is quite amusing to me. I know some people question the fact that she basically doesn't acknowledge that Rainbow Dash saved her life, but that really goes with the territory; in this sort of pseudomilitary setting, you can't let yourself show favoritism. ( A very subtle thing that I noticed during the episode was that when Spitfire was chewing RD out at the beginning and RD stayed stone cold giving the correct response, she gave a small smile; I'm sure she never forgot. This isn't a change in character, just another facet of her character.)