Interesting Facts

• Depending on how you draw your dividing lines, impostors are either omnivores or mesocarnivores, not entirely unlike bears; they require a diet of more than 50% meat and cannot be vegetarian or vegan without high technology, but can also digest plants, fungi, and so forth rather than only eating meat. The meat component doesn't have to be either eaten alive or from a sapient being. Most impostors who sneak onto other people's spaceships and start killing the crew have a motive beyond sustenance, although a few are desperate stowaways who ran out of other food they could eat.
  • This also means that an impostor impersonating a human and not sneaking meals elsewhere has dietary habits reminiscent of Ron Swanson from Parks and Recreation, of  'Give me all the bacon and eggs you have' infamy, albeit less inclined to turn up their nose at things other than red meat.
  • Artificially cultured meat such as vat-grown muscle tissue would work, but imitation meat with no animal tissue involved would have to be a really good imitation, better than your average early 21st century supermarket meat substitute. Low-quality meat that nevertheless came from an animal at some point is fine as long as it has the nutrients found only in meat, such as taurine and retinol; if it was that or starvation they could make up the difference with mechanically separated meat paste or cat food or something.
• That said, if you're already a murderer infiltrating a crew of humans by pretending to be one of them, it's theoretically easier to hide a body if you've eaten part of it first. And then you don't have to eat as much meat from the ship supplies when everyone sits down to dinner.
• "Impostors" is not the actual name of the species, nor does it sound similar, but their species is commonly known by that nickname outside the homeworld because of the whole shapeshifting and impersonation thing.
• It takes energy for them to shapeshift, but if they take their time and pace themselves when they're assuming a form they don't need to consciously maintain it afterwards, and if they're drugged, knocked unconscious, or killed nothing will happen to their current chosen form automatically. Quickly copying someone else's form does take energy and concentration to maintain and they have to change back after a few hours at most, not to mention that the in-between stage is an inhuman chrysalis-like shape that immediately gives away their nature to any witnesses. They can also modify their form to have more teeth and tentacles and similar natural weaponry if they feel so inclined, which takes less effort than changing their whole body but not zero. However, they don't have a single 'true form' underlying this that they can retreat to or be forced to revert to; what you see is what you get.
• Their species' usual means of reproduction is to bud off part of their body, which then learns to grow limbs and a differentiated head. The offspring doesn't retain most of the parent's memories or personality traits and generally behaves similarly to a human child. This is where mini crewmates come from! It can happen accidentally (resulting in a feral mini crewmate) if a severed limb gets left lying around.
• As a result, gender is optional for impostors. Some like it, some can't be bothered, but a single gender doesn't often become a major and indispensable part of their identity.