Final answer:
Mr. Carton demonstrates his observance in 'A Tale of Two Cities' during Charles Darnay's trial, where he notices their striking resemblance, resulting in Darnay's acquittal.
Step-by-step explanation:
The courtroom event that demonstrates Mr. Carton's high level of observance happens in Charles Dickens' novel 'A Tale of Two Cities'. Despite his seemingly indifferent and lackluster demeanor, Mr. Carton's attention to detail and sharp observational skills are evident during the trial of Charles Darnay. Carton notices the striking resemblance between himself and the defendant, which becomes a pivotal point in the trial, leading to Darnay's acquittal as the reliability of the eyewitness testimony is questioned due to this remarkable likeness. This incident proves Mr. Carton to be much more observant than one would assume based on his outward conduct.