In the book The Treasure of the Sierra Madre, there is an indication that Dobbs has already suffered a head injury, possibly during WW1. But the movie is different, with no omniscient narrator -- places where narration occurs in the book sometimes instead have Dobbs talking, as when he points out to Curtin that while an American may beg, he can't shine shoes in Tampico.
But Dobbs really starts acting strange and intensely paranoid, right after the cave-in. He was unconscious when dragged from the tunnel and mentioned that he was knocked on the head. I think he was pretty normal, even generously paying a part of Curtin's share to finance the expedition, before the accident. Is it implied that he was affected by the cave-in?
EDIT: To be clear, it seems almost like they went out of their way to show Dobbs as a reasonable man until almost immediately after the accident but it is not 100% clear. For example, he is kind of nasty to the little kid lottery seller but he does still tip him. He says excuse me to Curtin when he first meets him after merely brushing his foot.
One of the most significant indications showing Dobbs' basic decency/honesty is how he deals with the truly evil Pat McCormack:
McCormack is a contractor whose business plan is to simply get many workers to rig drilling camps and then simply stiff his employees -- of course, Dobbs and Curtin only hear of this after they have worked a couple weeks in the hot sun: PM strings them along, conning them on the ferry dock by giving them 10 bucks (probably in the 200-300 dollar range today) and telling them to meet him at a bar. They ask the bartender if he knows of PM and the bartender replies he comes in every once in a while. Another American overhearing this informs the pair that PM always does this to people who are ignorant of his approach to business.
The pair eventually leave the bar to go to a terrible "hotel" where the meet Howard for the first time. It is soon after this that they encounter PM on the streets with an expensive-looking female companion. It is of course clear from the woman and Pat's fine clothing that he has been paid but he continues to lie and at another bar, rather than giving them their wages, he (a big, well-fed man) attacks them.
This attack is a departure from what occurs in the book; he only calculates whether he can win against them and also if he ends up with blackened eyes he will lose his next contract. So, in the book, he disdainfully pays them.
But in the movie, there is a long fight that the two smaller men only win with difficulty. PM is laid out on the floor and when he says he can't see, Dobbs snatches the wallet he has been fumbling with and extracts only his and Curtin's wages. He would, just for the trouble PM has put them through, not to mention a severe beating even though they ultimately won the fight, be justified in the eyes of many in taking much more, like the whole wallet but he does not.
This departure from the book presents yet another instance of Dobbs being a normal, honest man, arguably more than normally honest.
After Howard is forced to stay with the Indians and now Curtin and Dobbs are trying to drive the mules with their (including Howard's) gold to Durango, we eventually see that Dobbs is 100% irrational:
Dobbs, perhaps triggered by the annoyance of having to handle Howard's donkeys, tells Curtin that Howard was foolish to have entrusted his gold to them and that they should not go to Durango to meet with the old man but rather take his gold.
Curtin is astonished and chides Dobbs, but this only makes Dobbs angry and paranoid: He now accuses Curtin of having the same idea but he also plans to kill Dobbs and take all the gold.
Curtin can say nothing whatsoever to Dobbs that convinces him that his intentions are completely honest. Dobbs now asserts that Curtin will kill him the first chance he gets.
Curtin first says, well, if you feel that way, we should split up. But Dobbs says something like, Oh, so you can then ambush me? (Curtin has a gun on Dobbs eventually, so could shoot him immediately but Dobbs replies, You don't have the guts to do it with me looking at you.)
Curtin offers every possible solution, importantly does not shoot Dobbs but Dobbs imagines motives no matter how contrived for why Curtin wants to kill him and steal the gold. I forget if the book has Dobbs acting quite this crazy. Curtin would be justified in shooting Dobbs, but does not and he lives to regret it.
What we can say for sure is, someone who acted like Dobbs eventually acted on the trail would be completely non-functional in society. If Traven /Houston wanted to show "gold fever," why have the head injury in the book or the concussion in the tunnel? What really is the point of an organic cause for the crazy behavior?
Perhaps Traven/Houston felt that a crazy person would show gold fever in an exaggerated way, the same symptoms but accelerated by the head injury. Even if he had managed to successfully sell the gold and get back to the USA, he probably would have killed someone out of paranoia anyway.
My question is then, did the film mean to imply that Dobbs was affected by the cave-in or was it the presence of the gold? Note that Dobbs displays not just paranoia but also greed we had not seen - note also, that despite initially being puzzled by Howard's desire to restore the mountain, he, even after the potential brain damage, goes along with the old man and all three men spend a week I guess filling in the holes they have dug, etc. (I wonder if that is a normal practice among miners?)
When Howard is first discussing in the Oso Negro Hotel gold prospecting, he warns that gold can make murderers out of honest men. But if this has been established, why have the cave-in at all? Besides the later completely bizarre behavior displayed while he and Curtin are on the way to Durango, Dobbs, almost immediately after the cave-in (which by the way, Curtin, at obvious risk to himself, goes into the cave to drag Dobbs out) begins to show paranoia, accusing his recent savior of wanting to find Dobbs' hiding place.
Of course, had Curtin wanted Dobbs' gold, he could have simply left him in the cave. (Curtin does in fact for just a moment seem to consider this, but almost instantly runs inside to save his partner.) I wonder why Curtin did not mention this when Dobbs actually drew on him after the accusation. Perhaps it seems too obvious to mention or a petty thing to do, but with a gun on me, I sure would have asked, Tell me why I dragged you out of the mine that day?
