If you're wanting to write fan fiction but aren't sure which of the characters are which, this may help. This information comes from the Rudnick and Singer books and cross-referenced with the IMDb.
The 10 Texas Rangers:
Besides Dan and John Reid, there are:
Blaine (Damon Carney) is the Ranger who makes the Redlegger comment.
Clayton (Kevin Wiggins) is the Ranger who makes the comment about running out of hanging rope. He is obviously named after Clayton Moore.
Collins (Leon Rippy)
Hollis (Lew Temple)
Martin (Chad Brummett) is the Ranger who makes the comment about Butch eating someone's eyes.
Navarro (Robert Baker) is the Ranger who makes the comment about Latham making an example of Butch.
The two rangers who are killed by Butch on the train at the start of the movie are Boss (W. Earl Brown) and the other who is played by an uncredited actor.
The Cavendish gang:
Wayne Barret (James Frain) is the one who resembles The Man With No Name.
Butch Cavendish (William Fichtner, and for the flashback, Travis Hammer)
Frank (Harry Treadaway) is the cross dresser.
Jesús (Joaquin Cosio) is the Hispanic man at the Reid farm.
Kyle "Skinny" (Matt O'Leary) is the thin man with the long curly beard.
Ray (Damon Herriman) is the man who goes into the mine with Skinny. He has a scar around his neck as the result of a failed hanging and wears the hanging rope around his neck.
At Red Harrington's Traveling Entertainments:
Homer (Leonard Earl Howze) is the bouncer.
Rosalie (Laina Loucks) is the woman who flirtatiously says, "Hi, Tonto."
The cast list includes women named Glenda (Joanne Camp), Helen (Tina Parker) and Jane (Allison Marie Volk), who are three of Red's "professionals."
Big Bear (Saginaw Grant) is the chief.
Red Knee (Gil Birmingham) is the man who throws John into the tepee.
The warrior with Red Knee is unnamed (played by Malachi Tsoodle-Nelson).