Adaptational Villainy: Terry Lennox IS the murderer, after all! While the book didn't exactly have this character come across as particularly clean by the end, still, lying to your friend and letting him deal with the consequences still isn't as bad as murder....
Adapted Out: Most blatantly, Linda Loring—the classy yet just-as-snarky-as-Marlowe Spirited Young Lady whom Marlowe meets in the book during his investigation. She's the sister of Terry's late wife, and shares Marlowe's doubts that Terry killed anyone—and thus, becomes Marlowe's ally and uneasy assistant in his investigation. Chandler's purpose for her was to be the "Princess In Sour Dress" to Marlowe's Knight in Sour Armor—and their parting near the book's end forces the detective to begin examining his love of isolation. In the movie, she's nowhere to be found, and instead Altman has Marlowe strike up a complicated relationship with Mrs. Wade—who, ironically, was the murderer in the book.
Ambulance Cut: Marlowe runs into the street, gets hit by a car, and we cut to the ambulance.
Angry Guard Dog: The Wades' dog is never violent, but it is always barking angrily whenever Marlowe is around.
Appeal To Worse Problems: A variation of the "starving children in Africa" argument: when the cat doesn't want to eat, he says, "What about all the tigers in India they're killing because they don't got enough to eat?"
Butt Monkey: Marlowe as part of the deconstruction of private eyes. He's a man who lacks a Friend on the Force as the police have no idea who he is. He is no ladies man, as the girls next door make fun of him and the villains frequently get the better of him in fights. He also loses his cat.
Cool Car: Marlowe's 1948 Lincoln Continental convertible
Covers Always Lie: The DVD cover shows Marlowe holding a Beretta 92SB, even though it didn't even exist when the film was made, and a poster has him holding a Colt Detective Special with the Tag Line "Nothing says goodbye like a bullet", a line from an early script that was never incorperated into the final movie, yet he uses a Smith and Wesson Model 10 at the end.
Cut Himself Shaving: Marlowe notices a bruise on Eileen's cheek, and says it doesn't look like she walked into a door. She says that she didn't. She fell out of bed.
Deconstruction: Supposedly Altman wanted the film to be one, Up to Eleven, of the whole "Private-Eye" genre—to the point where it would actually more or less put an end to these kinds of movies. Didn't happen, of course.
Destroy The Product Placement: The hero is interrupted by a gangster who is accompanied by his goons and his lovely mistress. Said mistress interrupts the gangster's rant, by informing him that she’s thirsty and would like a Coke. One of his goons fetches an open bottle from the refrigerator. The gangster swigs from it, complains that it’s flat, and then swings it into the mistress’ face, causing it to break and leaving her in pain.
The Film of the Book: Although it changes the time period and the identity of the killer in the end. And Marlowe's One True Love in the book, Linda Loring, is noticeably absent—apparently to make room for a more "fleshed-out" Mrs. Wade.