This scene is notable in that it combines many of Leone's trademark techniques.
The use of image and sound montage:
While film editing is usually approached in a way to make the transition from one shot to the next as fluid as possible, and at the least noticeable moment, the montage technique employed here by Leone has the opposite effect. We are intentionally aware of the rapid cutting, punctuated further by the gunshots and jangles of the soundtrack.
The way characters enter the frame:
With the camera stationary to the scene a character enters in close-up - in this case 'The Man With No Name' enters from the left, his gun hand dominating the frame. We know it's him from the wrist protector.
This is a device that Leone would use to more startling effect in later movies, particularly the first scene of 'The Good, the Bad, and the Ugly' where a long shot of a barren landscape is suddenly eclipsed by Al Mulock's face in extreme close-up.
This deep focus juxtaposition of wide sweeping landscapes with close-ups of the characters is used throughout the entirety of 'Once Upon a Time in the West'.
The use of the close-up:
Not just close up, but extreme close-up - right into the eyes of the actor. This is the technique that Leone is most renowned for. He used it time and again but probably never better than in the three-way showdown of 'The Good, the Bad, and the Ugly.'
The point-of-view shot:
When we look at the wanted poster, it is through the eyes of the characters. A striking example of Leone's use of the point-of-view shot is when Ramon Rojo is killed at the climax of 'A Fistful of Dollars' - the camera points skywards, spins and topples. A similar moment occurs in 'The Good, the Bad and the Ugly' where a woman faints on discovering her family slaughtered. Other examples are the flashback sequence of 'For A Few Dollars More' where we see the couple on the bed from Indio's perspective, the flashback sequence in 'Once Upon a Time in the West' where we watch Frank emerge from the haze through Harmonica's eyes, and in 'Duck You Sucker' when we see the clouded image of Juan and his family through Sean's motorcycle goggles.
The comic-book moment:
Almost cartoon-like, certainly surreal.
Other examples are the 'Golden haired angel watches over him' instant in 'The Good, the Bad, and the Ugly' where a heavenly chorus punctuates the soundtrack. And moments in 'Duck You Sucker' - when Juan sees Sean as his ticket to robbing the bank at Mesa Verdi it's name lights up over Sean's head - when he sees the bank for the first time it shines bright gold.
Character and plot development
The scene also important in that it develops the story and gives character insight.
Having been introduced to the characters individually in previous sequences, this is the first time that the three protagonists are brought together - they do not actually meet but their stories are now inextricably tied. It is not until further into the film that the characters finally meet after we have again returned to their individual story development.
But the scene also gives a clue to the motivations of the two bounty killers. The Man with No Name's eyes move upward to the reward money. The other bounty killer's eyes move down to read 'Dead or Alive'. The shots that ring out make it clear that 'Alive' will not be an option. And the rapid editing between the eyes of the bounty killer and the bandit suggest a deeper purpose - a personal vendetta - the settling of scores. Although we may not realise it yet, the outcome of the film is already sewn up in this scene.