John Marston makes his first and last meeting with Müller when he sees him playing in a Poker game with Landon Ricketts, Manolo Santander and The Stranger at the Saloon in Chuparosa. Müller appears to be on tilt after Ricketts rakes in a large pot, during which Ricketts offers John to join the game, who promptly does so. After a few rounds, with Müller making comments hinting that he believes John and Ricketts are colluding, John glances over at Müller and then looks down at his own cards. He then accuses John of looking at his cards, calling John "a fucking cheat", with John denying the allegations and both him and Ricketts telling Müller to calm down. The situation escalates to the point where Müller then draws his gun and stands up, aiming it at John. As soon as he pulls out his weapon, John and the others at the table stand up and aim theirs as well. John has his aimed at Müller, while The Stranger has his aimed at John. Ricketts has his weapon aimed at Müller, with Manolo Santander aiming at Ricketts. And Rigoberto Artiz is aiming his gun at Manolo Santander. They reach what Ricketts calls an impasse (what would today be called a "Mexican Standoff"), and eventually, Müller decides that he and John will Duel for the chips in the pot with The Stranger as his second. Like with all compulsory main storyline duels, there is no way to disarm him. After instruction from Ricketts, John makes quick work of Müller. John and Ricketts then go to have a drink, with Ricketts saying that "Mr. Müller is buying".
Marston calls him "Germany" when he tells him to calm down. It is fairly similar to Irish, French and Welsh.
Andreas says the word "Fuck" more times in one sentence than anyone else in the game.
He appears to have a grudge or hatred towards Americans, particularly when he says "No one steals from me, especially an American!". He also calls John a "Yankee" several times. He claims to have no problem with Landon Ricketts, although this may be because Ricketts is a legendary gunslinger and Müller doesn't want Ricketts involved.
The player can kill Müller while he is walking to the duel spot, but they will fail the mission.
Very rarely, there is a glitch where, when Müller is shot on the way to the duel spot, the characters around Marston freeze. The only way to escape this is to fail.
His name could very well be a reference to German footballer Andreas Möller. This is very likely since Möller was well known for his constant whining and complaining about being roughed up by opponent players and being mistreated by the referees.
The player can actually cheat against him by using the Elegant Suit, but no money will be awarded.
Sometimes, Müller will carry a Semi-automatic Pistol to the duel. However, once killed, he will drop the Schofield Revolver.
At the end of 'Lucky in Love", Landon remarks "I doubt Müller will be missed." In the following newspaper, Müller's death receives both an article and an obituary.
Strangely, the newspaper article released after his death says he was a shining citizen in Chuparosa, which doesn't make much sense, as from what you see of him in the game he seems rude and nasty to most people, most notably John and Ricketts.