Trivia: In the scene where Lenny is on the phone describing how Sammy Jenkis ends up in a home due to the death of his wife, it shows Sammy sitting in a chair looking towards the camera. Sammy is temporarily blocked from our view by someone walking in front of him. As the person passes by, it is not Sammy that re-appears, but Lenny. To see it well, you will have to use the frame-by-frame mode. There are about 5 visible frames of Lenny in the home, and then it cuts back to him inside the motel room talking on the phone again. (01:29:55)
Trivia: The movie plays out in alternating color and black and white scenes. Chronologically, the black and white scenes are first, in the order they are in the movie, and the last black and white scene fades into color because it is perfectly continuous leading in to the color scenes.
Trivia: Carrie-Anne Moss recommended Joe Pantoliano for the role of Teddy.
Answer: It is never explicitly given. The most Leonard says on the subject is: "I'd rather be mistaken for a dead guy than a killer." Speculations include (you can make up your own motives as well) : (1) The clothes and car are so much nicer than his. If you are willing to kill someone: stealing is not really a "crime." Why not take the nicer objects? (2) It could be part of his "routine": Kill a man, take his clothes and car. The clothes he had on and the truck may be from the man he killed a year ago. (3) It could be that he wants to make the killer of his wife suffer even more, and takes his clothes as a way of humiliating him. Leonard takes the man's life-his clothes and car, which are wrapped up in his identity-just as the man took his. This idea seems to work with a theme in Memento about "Identity" (especially mistaken identity). Natalie thinks Leonard is Jimmy, then thinks he is Teddy, then learns he is Leonard. Teddy is "mistaken" for the second killer, Jimmy is "mistaken" for the 2nd killer. Sammy's story as a part of Leonard's story, etc. (4) It could "simply" be explained as a "plot device": Leonard has to do it, otherwise he won't find the note in "his pocket" and meet Natalie. (5) Leonard doesn't want to admit he's a murderer. He's lying to himself. If he's the victim, then he cannot be the murderer. (6) Leonard takes Jimmy's clothing as part of his routine of killing J.G.'s he becomes another person, he's the victim not the killer, thus "I'd rather be mistaken for a dead guy than a killer." and that's why he also takes his car, so he has to, once again, find his wife's killer and kill him.