These shapeshifters - known as Doppels - aren't the stereotypical "blob of goo that becomes whatever it wants" shapeshifters. Instead, they're humanoids that can assume the physical appearance of another human, usually to steal their identity. What is the most plausible (or the least rule-breaking) way for them to do this, biologically speaking?
Some parts of this have obvious solutions. Color changes are a no-brainer - many animals use modified chromatophores to change their skin color, and it seems reasonable that they could also be used to change eye color. Hair changes can probably be accomplished using a way to grow it extremely fast, and then simply cutting and dying it (but if there's a way to do it purely biologically, let me know). Changing their gait and voice could be done with nothing more than good control of their movements and speech. What I haven't been able to figure out is how they could change their actual body shape.
Here's the specifics. Doppels can:
- Alter their height and the length of their limbs, up to an inch or two in either direction.
- Alter their facial structure to assume a wide range of faces, as long as the face has no major deformities (such as a missing eye).
- Alter their apparent weight within a moderate range (they can't appear malnourished or extremely obese, but most humans are within their range).
- Alter their muscle definition within a similar range.
Doppels can't change their apparent sex (unless having their other abilities would already let them).
I haven't decided how long the transformation takes, mainly because it'll be based on these answers. Doppels usually assume a person's identity for months or years at a time, so it doesn't have to be a quick and easy process, but ideally they'd be able to transform within a few weeks at most.