Robert Mitchum gets a few things very right about the Marlowe character; his weariness, his exhaustion, and his dogged determination despite those things. He doesn't quip as fast as he should and Marlowe's sarcastic wit sort of falls dead after leaving Mitchum's mouth.
If it wasn't for the nudity you might think this was a made for TV movie. The pacing is a little slow and the screen writers decided to monkey with Chandler's plot for some reason. I mean it wasn't like Chandler's plots are some exquisite puzzle of perfect logic, they aren't. Often they are just confusing and meaningless. Often Marlowe doesn't even know what is going on. But I feel like if you're going to change the plot it should be motivated somehow to make the movie shorter or simplify things. But they add characters and sort of bungle and mix up the ending.