One type of transformation occurs when a character throws off a disguise. Disguises themselves fall into various categories: disguises assumed by need, disguises assumed by personal desire.
One disguise is a type that Agatha Christie used in her writing: the disguise that occurs because of what people don't see, based entirely on their assumptions. (Ignoring "staff," for instance.)
Research backs up Christie, including research about eyewitnesses and whether or not people notice gorillas during basketball games. The disguise is not as fool-proof as Christie often claims. In the aforementioned research, what people don't see is a percentage, not an absolute.
But her point is still valid. When I was about ten and playing a softball game in the gym with other ten-year-olds, I walked, rather than ran, between two bases. Only the amused teacher noticed what I was doing. If I had run, I would have instantly attracted everyone's attention, which was currently focused on the batter. So I walked and made the base.
Disguises that blend in because nobody expects what is there are fairly brilliant.
From the book version of Appointment with Death, Lady Westholme doesn't dress herself up as someone else but she does use her companion's assumptions to disguise the truth. When Lady Westholme calls out to the dead woman and then mutters, "So rude, the way she grunted," her companion assumes that an entire dialog took place. Consequently, the companion reports later that the dead woman was actually alive at a certain time.
People who see half a stop-sign draw the whole thing. True story--and a remarkable indicator of how our brains fill in gaps.
And how Agatha Christie was able to devise murders that others might miss.