MacMillan

"Good Shot Lieutenant, I think you blew off his arm. Shock and blood loss'll take care of the rest."

- MacMillan to Lieutenant Price, upon the latter shooting Imran Zakhaev

The player controls Price during "One Shot, One Kill" and "All Ghillied Up". However, the player cannot finish the mission without carrying Captain MacMillan to safety in "One Shot, One Kill".

Biography
MacMillan is a Scottish Special Air Service officer featured in Call of Duty 4: Modern Warfare. Fifteen years prior to the modern events of Call of Duty 4, he was a captain of the SAS and the commander of a two-man SAS sniper team, including then-Lieutenant (pronounced left-tenant in the British Armed Forces) Price, who was assigned to assassinate Imran Zakhaev in a covert operation in Pripyat, Ukraine. After Price successfully injured Zakhaev, MacMillan and Lieutenant Price were compromised and forced to flee. MacMillan's leg was injured after a Russian gunship crashed and nearly killed him, so Price had to carry him the rest of the way to safety. Captain MacMillan is famous for his thick Scottish accent. During the mission One Shot, One Kill, and All Ghillied Up, MacMillan instructs you throughout the levels.

Quotes
"Goodnight ya bastard..."

- MacMillan after shooting down the helicopter with Lieutenent Price in "One Shot, One Kill"

Trivia

 * Captain MacMillan is voiced by actor Zach Hanks.
 * In the lyrics of Deep and Hard, the song played in the credits, MacMillan is mentioned in the following phrase: "... This ones for MacMillian, 15 years you had relaxing..." These lyrics point to the fact that he retired as a Captain, probably because of his leg injury and age.
 * Before you get to the main menu, a cutscene plays, saying " 50,000 people used to live here, now it's a ghost town." If you listen closely, Zach Hanks does not voice Macmillan here. It sounds more like Gaz. Furthermore, when MacMillan says this in All Ghillied Up, he says "50,000 people used to live in this city", the voice was possibly changed for international audiences the developers thinking they won't understand the accent
 * In sniping missions in which the player controls Soap, Captain Price will occasionally say, "Good job, MacMillan would be proud."
 * There is speculation that MacMillan is named after the McMillan Tac-50 sniper rifle, which was used to get the farthest recorded sniper kill.
 * If you look closely when MacMillan gets hit with the fallen gunship on One Shot, One Kill, the helicopter doesn't even touch Macmillan when it crashes. This may be just a animation error, or it might just have portrayed as him falling while running away, and in the process he broke or badly sprained his leg. It is most likely the latter, as it was once stated that the helicopter scene had taken a considerable amount of time to create, and the developers would most likely not overlook it.
 * Strangely, in some trailers he is seen with a suppressed M4A1 and other with his M21. It might be he was originally supposed to have an M4 but they changed it and gave him a sniper.