The casting may seem odd with the decidedly American Harvey Keitel and Keith Carradine in the roles of two French men. Of course if they were two British actors, Michael York and Olivier Reed were apparently director Ridley Scott's first choice, no one would bat an eye even though that's hardly being French either. The casting though I think only seems odd before you actually watch the film and see how their performances actually work in context to the film. Anyway neither man puts on a French or even English accent in the film. They do both adjust their accents though which is a good thing since Keitel's natural accent is particularly strong. Keitel doesn't cover up his accent though but rather all he does is ease back on it, as does Carradine. Keitel just sorta takes away the sharper edges of his usual accent and actually is quite effective in overcoming his apparent miscasting.
Carradine's performance acts as a great counterpoint to Keitel's as he shows d'Hubert to be in many ways the opposite of Feraud, although they both believe fervently in their own personal sense of honor. Carradine makes himself kinda the straight man, and easily the more sympathetic of the two as he portrays d'Hubert as an easygoing guy who's rather charming a low key sort of way. Their initial face off when d'Hubert gives the message to him is quite memorable. Keitel's terrific in the basically foolish passion he portrays in Feraud as he demands a battle to the death immediately over the insult d'Hubert gave him, even though d'Hubert was only doing his job. Carradine does quite well by merely portraying d'Hubert's reaction as what any sensible man's reaction would be. Carradine's disbelief is perfect in basically setting up how d'Hubert whole association with Feraud is one of obnoxious obligation.
Keitel and Carradine were perhaps perfectly cast actually since the way they handle this feud works so well in context to the film. There is a certain comedic bent about their performances that never makes the proceedings seem stuffy in the least, as they might have with less interesting actors. I love how Keitel does not really portray Feraud as really even hating d'Hubert in anyway. In his scenes he has not really a death stare toward d'Hubert at any time but rather something much stranger. It never feels unnatural though as Keitel shows this almost insane, yet he is so proper about it in his gentlemanly way, devotion in Feraud to his code of honor. It is in this code where Feraud wants to apparently kill d'Hubert not for hate of d'Hubert as a man, but hate directly at the slight insult that d'Hubert once gave him. Keitel makes this drive in Feraud a force of nature and something that always seems perfectly sane for a man like Feraud.
Carradine on the other hand is equally good by portraying d'Hubert reactions to every duel in a far more normal fashion. He does also bring a passion in terms of the idea of honor, but Carradine makes it less vicious so to speak. Carradine is very good in portraying such a quietly growing desperation d'Hubert as he has to keep fighting Feraud despite trying his best to avoid him using various loopholes in the dueling guide such as not being able to fight in times of war or when the men aren't the same military rank. Carradine is very good by just playing it close to the chest and giving a pretty moving portrayal of a man forced by circumstance to keep risking his life. It isn't in big speeches where d'Hubert expresses this but rather just simple reactions. Carradine makes the most of these by just showing the honest fears of a man who has to fight to a possible death simply because he was the guy who had to deliver a message one day.
Both actors are fantastic in the actual dueling scenes. It is rather amazing just how good they are because they do more than you would expect in the scenes. On one end physically they are both incredibly believable in just displaying the physicality of each fight. Their middle fight of endurance is particularly good as both men show their characters are at complete exhaustion. The two are so good in giving tension to the fight in portraying such genuine reactions in the two with Carradine bringing a little more obvious concern and Keitel more ferociousness. What I love best perhaps though is that they do manage a certain lightheartedness within the tension. They both are so careful never to compromise the severity of the duels though. They are entertaining though by carrying themselves always with the reserve of a proper gentleman and soldier even though their lives are on the line.
Each actor realizes their character so well to make the ending rather remarkable. Carradine makes d'Hubert someone we can naturally root for and makes the happy ending feel completely appropriate. Keitel is equally good by never allowing Feraud to seem to be even a villain. It is rather his pride and duty which Keitel creates such a strong sense of in his character. This allows for his character's ending to be surprisingly moving as he shows a man basically looking into the void as he know is a man whose lost his purpose in life. Unlike Ryan O'Neal in Barry Lyndon, who also played a duelist, by the way here's two more guys who would've been better in that role, they completely overcome the fact that they seem miscast on paper. The main way they do this is by just giving a damn good performance. They make the duels more than just a series of physical clashes, but as well a consistently fascinating clash of personalities.