This Robert Mitchum vehicle seems to be inspired by the same line of reasoning that produced the 1997 12 Angry Men telefilm starring Jack Lemmon: gee, our lead would have been great in this role 30 years ago. It aims to be more to eliminate the prefix from neo-noir—unlike The Long Goodbye (made two years earlier).
The Long Goodbye is, I think, a better novel than Farewell, My Lovely—which may explain some of the films’ difference in quality. But more than that, Farewell, My Lovely is just plain uninspired. The film is a little tarted up from the Chandler novel, but little value is added. Robert Mitchum delivers his voiceovers with stony expertise, but his acting seems indifferent.
If you’re a Chandler completist, see this film. If not, see Murder, My Sweet instead. It is a far superior filmed version of Chandler’s novel.