Fire
Fire seems to symbolize climaxing action and confrontation; at the first Hay on Fire festival, the bonfire provided a backdrop for the dramatic action and disappearance of Cassiel. In the later festival, it acts in the same way; it even comes out that the fire was the agent that actually killed Cassiel in the previous festival after Frank bound him and shoved him inside the Wicker Man.
Additionally, the accident that separated Damiel from his family was a house fire.
Business Clothes (Frank's Attire)
When a character in the novel is dressed in Frank's suave style, it illustrates a façade of deception. Frank himself wears such clothes as part of his essential character, and he is secretly a criminal. When Chap wears Frank's clothes, he is tacitly agreeing to maintain the deception of his identity, pretending to be Cassiel when he actually is not.
Excessive Cleanliness
Related to the clothing façade, when something is described as being abnormally tidy, it is usually hiding something dark. Cassiel's laptop, having been wiped, is excessively clean, and that reflects the dramatic occurrences of Cassiel's disappearance. His tidy room, similarly, hides a darker secret: Frank went through his things and disposed of them, trying to cover his tracks.
The Cage
When Chap describes his life as Cassiel as a cage, he is referencing the lack of freedom he feels while attempting to impersonate someone he isn't. The imagery of a cage reinforces that impression; he can do nothing to escape the role he has chosen to play, and he can't let up his acting even for a minute when in the company of others.
