This film is a sharp-edged study of the effects of greed on otherwise normal men, and one man in particular: Bogart's Fred C. Dobbs. Dobbs and Bob Curtin (Tim Holt) are down and out and meet up with prospector Howard (Walter Huston). When Dobbs wins a lottery, he uses the proceeds to finance a trip for the three to central Mexico to search for gold.
The three have to deal with the lawlessness of central Mexico at the time - bandits were actually on the loose in that country killing anybody with stuff, and taking that stuff. The Federales were a violent solution to a violent problem - killing the bandits after a summary judgement and the bandits having dug their own graves. So our trio not only have to worry about bandits once they strike gold, they have to worry about the darkness of their own souls.
In the beginning, Bogart's Fred C. Dobbs is a decent guy who does not take advantage of others. Dobbs only takes his money from the guy that wouldn't pay and he does share his lottery ticket and is generous with his fellow miners, but as greed begins to take root in him, little by little we see his goodness eaten away. It's a great credit to the writing and Bogart's skills that this is done gradually and played out over time. Incidentally, that's director John Huston "staking him to a meal". One of the best director cameos ever (although Polanski in Chinatown is equally great)!
Dobbs overestimates himself and the fallibility of human nature. Walter Hustons character freely admits what gold could do to any of them including himself. Dobbs is sure it will never happen to him, but he's never had anything, so he's never faced temptation, and when he falls it's a long way down.
This may be Tim Holt's finest performance - it was probably his finest opportunity given he had spent years laboring as a B western star on the RKO lot. Walter Huston as the prospector, minus his dentures and plus a bunch of pounds and with holes in his clothes is not the debonair fellow you are used to seeing in film . If Mary Astor's character in Dodsworth could have thought this was the future appearance of the man she loved, would she have taken her gondola in the other direction? I guess we'll never know.
Highly recommended as one of the great character studies in which several characters get studied in detail.