Derek's repetition of the line "But why male models?" for the second time, in the cemetery scene, was an ad-lib by the actors; Ben Stiller forgot his line and simply repeated the earlier line again, and David Duchovny ran with it and gave him a straight answer.
Derek Zoolander's "Blue Steel" look evolved from Christine Taylor ribbing her husband, Ben Stiller, about the way he combs his hair in front of a mirror.
Owen Wilson (Hansel) wore a wig for the entire movie. He was filming Atrás das Linhas Inimigas (2001) at the same time, and had to keep his hair short for the role.
Ben Stiller wrote the part of Hansel specifically for Owen Wilson, and said that no one else was even considered for the role.
Ben Stiller wrote David Bowie's scene as judge of the walk-off into the script without knowing for sure whether he would agree to it.
Vince Vaughn: As Luke Zoolander, Derek Zoolander's estranged brother who works in the coal mines in southern New Jersey. Vaughn gets almost as much screen time as Jon Voight and Judah Friedlander, who play Derek and Luke's father Larry and brother Scrappy respectively. Unlike Voight and Friedlander, Vaughn has no lines and is uncredited.
Ben Stiller: [Star Trek] Mugatu is named after Mugato from A Private Little War (1968). His costume was also modeled after the character. Note that the 'Mugato' in the episode is credited as 'The Gumato'.