Stevenson really uses indirect characterization to help paint more a detailed picture of Jekyll and Hyde, and, perhaps, their relationship.  Jekyll looks "deathly sick" and his manner is "feverish."  When Jekyll declares that he does not care what becomes of Mr. Hyde, that he is completely finished with the man, Mr. Utterson is "surprised at his friend's selfishness," and thus readers are given to know that Dr. Jekyll is not typically a selfish man; he is usually very kind to others.  

Further, the letter purportedly written by Mr. Hyde to Dr. Jekyll intimates that he has been unworthy of the doctor's generosity and that the doctor should not worry himself about Hyde's ability to escape.  The narrator says that this letter "put a better colour on the intimacy [between the two men] than he had looked for [...]."  Thus, we (and Utterson) learn that the relationship between Jekyll and Hyde is not based on blackmail or anything else of that nature.  

However, Dr. Jekyll said that the letter had been hand-delivered.  But when Utterson asks Poole, the butler, he says that nothing has been delivered except by post, and so this makes it seem as though Jekyll isn't being completely honest, and he seems to have something to hide (no pun intended).  Moreover, the fact that the letter seems to be written in Jekyll's own hand is further evidence that he is not telling the truth, that he is, perhaps, not the man everyone believes him to be.

Thus, through indirect characterization, it begins to seem as if Dr. Jekyll, especially, is not the same man, motivated by the same things that he has been in the past, and that he is not who he claims to be.  He seems to have much to conceal if he is willing to be so dishonest as to lie baldly to his lawyer and friend.