Do not pretend to be another character when you are IC or OOC. Characters who are not being played fall under the emitting rules.
There are certain situations where you may spoof (play) a character belonging to another player. If the player wishes for you to finish a scene for them, they simply have to give you verbal permission to do so. If you are to be gone for an extended period of time (a few days) and wish someone to emit for you while you are gone, then permission needs to be +request(ed) of staff so that we are aware someone else may be acting for your character.
Spoofing also applies to @emit ting in a room and pretending that it's someone else saying it. This can become very annoying, very fast. Don't do it.
Additional note. Characters who shape shift or copy other's appearance, it's courtesy to talk to that person copied before it - recommended, not required. Keep it IC and within character limits if doing so.