One distinct similarity between both book and film is the establishment of the elements of the hardboiled detective world. What the book explores as the underworld that Philip Marlowe must navigate is something that the film is able to establish as a fairly strong element of the film noir genre. For Marlowe in both representations, finding and establishing "truth" becomes very difficult because the world is filled with inauthentic and duplicitous people.
Another similarity between both book and film is that those who commit wrong end up facing punishment. There is a sense of justice having worked itself out in both. Those who do wrong end up facing punishment, by either their own hand or through external law enforcement. In both the book and the film, Marlowe is the "last man standing," almost serving as testament of the toughness needed to survive in this life.
A significant difference between both would lie in the moral characterization of Marlowe. The film version of Marlowe in Murder, My Sweetis much more of a redemptive character than in the book, Farewell, My Lovely. For example, the movie opens Marlowe's narrative with him actively helping Moose Malloy locate Velma. Marlowe is an agent who helps Moose. Yet, in the novel, Marlowe is pursuing a dead- end case and only picks up the pursuit of Velma because Nulty is not competent to realize how to do so. The book has him as an agent of opportunity, whereas the film shows him as actively engaged in a self- initiated quest. Such a difference highlights how Dick Powell was viewed as the "eagle scout" Marlowe, something that was active in the director's mindset:
Dick Powell fit the character as far as I could see. After all, what is Marlowe? He’s no Sam Spade. He’s an eagle scout among tough guys. He’s a moral, ethical man, with a strong sense of responsibility.
This notion of constructing Marlowe on screen as a pillar of morality was not as evident in the book, where Marlowe was less of an active agent in the world around him.
Part of this enhancement of Marlowe's character on screen involved making the world around him quite formidable. The translation of Chandler's protagonist to screen shows him besieged with a world around him that is difficult to navigate. The opening where Marlowe is temporarily blinded is a significant difference than what is seen in the novel. It's almost as if Marlowe's blinded state only heightens his commitment to morality and ethical conduct. Showing Marlowe as drugged, beaten, and held captive are further examples that display his "strong sense of responsibility."
Some of the cinematic techniques enhance this. One such technique is the use of voice- over. Going into Marlowe's mind through voice- over is a difference than the novel. At the same time, throughout Marlowe's enduring all that besieges him, he never wavers in being "potent and convincing" as the one force of good in a "cynical vision of society." Characterizing him in such an exalted manner is not so evident in the book. This difference can be seen in the ending of the movie, where Marlowe gets the girl as opposed to the book where Velma kills herself. A significant emphasis of the film is to show Marlowe as a force of redemption in a world that lacks it. This is not as demonstrative in the book, which Chandler saw as a construction of his writing "style" and not something that featured as directly of a moral narrative as was featured in the film.