A motion picture extra is a person who is hired to appear as part of the background of a film — for example, as one of a crowd. These people generally do not have speaking roles, and are seldom given screen credit. In return for a per diem payment, an extra must follow the same directions for as many takes as are necessary for a particular scene, until the film director is satisfied. As with the first two installments of the Back to the Future trilogy, there were hundreds of extras hired for Back to the Future Part III. Besides the many residents of Hill Valley in 1885, there were American Indians who pursued Marty, followed by the U.S. Cavalry, and a few persons whom Marty encountered on his return to 1985.
The scenes from 1955, at Courthouse Square, Doc's mansion, Boot Hill Cemetery, the Delgado Mine, the City Archives, and the Pohatchee Drive-In contained no persons other than Marty and Doc, although other people are visible in photographs. A car can be seen driving past the Pohatchee theater, although the driver is too distant to be seen.
September 2 (Wednesday)
- Among the Indians and the U.S. Cavalry, all of whom are on horseback, are standard bearers for both sides, with an Indian holding aloft a feathered pole, and two U.S. Army men carrying (38 star) flags. A bugler can be seen and heard, and a commander yelling "Charge!!" can be heard as well.
- After Marty falls down the hill and lands at the McFly Farm, Seamus McFly is the first on the scene. In the background are several farmworkers, presumably employed by Seamus and Maggie.
September 3 (Thursday)
- When Marty arrives at the Hill Valley Railroad Station, he can see a conductor in uniform, who is standing outside the window of the Western Union counter. As Marty walks past the counter, the man vanishes between shots.
- As Marty walks up Second Street, the first person who notices him is a man on horseback.
- Two men are leaning on a fence at "Honest Joe Statler's" livery stable, and picture of Statler is on the sign.
- At a bath house, a cowboy in a bathtub says, "Give me some soap, Wang," and a Chinese man responds, "Uh, here you go."
- A butcher chops the head off of a chicken.
- Another man on a horse says, "Easy, boy!" as he rides past Marty (he says it to the horse, not Marty).
- Two women can be seen talking and watching Marty as he passes the poultry shop.
- A man with a hammer is working at the cabinetmaker's shop.
- Men are standing on the balcony of the Palace Saloon.
- A. Jones can be seen shoveling manure.
- A Wells Fargo stage, with six horses pulling it, goes by. In front are the driver of the horses and, next to him, the guard who is literally "riding shotgun".
- A cigar maker watches Marty
- A man is whittling out in front of the Palace Saloon.
- A woman and her daughter cross the street toward the courthouse site as Marty approaches the saloon.
- Unlike Lou's Cafe, the Palace Saloon has a second floor (for a hotel). One of the three prostitutes watching from the balcony is script supervisor Marion Tumen-Jones.
- Besides the three Western codgers (Dub Taylor, Harry Carey and Pat Buttram), Chester the bartender (Matt Clark and Buford and his gang, there are about ten other people in the bar. A man sitting near the front is reading a newspaper, and he spits tobacco juice into a spittoon.
- After Marty runs from Buford, he dashes past a man standing outside the saloon, passes two men on horses, and has to dash around two women.
- A horse rears as Marty tries to get away, and his rider yells "Whoa!"
- A man driving a cart with two horses watches as Marty gets lassoed and pulled through the street
- Four cowboys watch from the roof of the Palace Hotel
- A man in front of the livery stable watches (and presumably alerts Doc Brown to what's happening)
- Several men are standing on scaffolds, working on the construction of the Courthouse.
- Several people standing in front of the Hill Valley Telegraph office, and one of the construction workers, watch as Marty is being hanged, and as Doc shoots the rope with his sniper rifle.
September 4 (Friday)
After realizing that a train is their only hope of pushing the DeLorean to 88 mph, Marty and Doc talk to the train engineer (Bill McKinney at the station. Among the passengers waiting to board the train to Haysville are a woman with two children. Clara has arrived on the same train, but Doc forgets to pick her up as he had promised. Later, Doc rescues Clara from certain death in the Shonash Ravine.
September 5 (Saturday)
- At the Hill Valley Festival, Hubert, the mayor (played by Hugh Gillin), dedicates the new clock at 8:00 pm.
- A photographer, identified as R. Nelson takes the picture of Marty and Doc. In the prior timeline, Doc alone had been in front of the clock. Nelson is played by Dean Cundey, the director of photography.
- Behind the drummer and the guitarists portrayed by the members of ZZ Top are a number of fiddlers and other local musicians.
- The Colt firearms salesman (identified in the novelization as Elmer H. Johnson) is portrayed by Burton Gilliam.
- Besides Marshal James Strickland, the deputy marshal (Donovan Scott) and Strickland's son (Kaleb Henley), there's the man who keeps the checked firearms. He looks up suddenly as Buford throws a knife in the counter.
- Among the more noticeable persons who watch Doc and Clara dance are a hat-wearing lady in a brown striped dress; like the others, she claps in time to the music.
- The square dance caller is portrayed by James A. Rammel
- Doc and Clara change partners with another couple during the square dance
- As Buford's gang walks toward the saloon, one of the gang steals a bottle of whiskey from a passerby, who exclaims "Hey, that's mine! Gimme that!"
- A woman credited in the film as the "Pie Lady" hands Marty the Frisbie Pie Company plate that he will use to deflect Buford's gunshot. She is played by Jo B. Cummings.
- When Clara kicks Buford in the shins, she's watched by a man with a bowler hat, and his wife, who has a straw hat.
- Standing behind Seamus and Maggie at the dance are a lady wearing glasses (one of the few bespectacled characters in the trilogy), and a man wearing a string tie.
- Doc's hat is knocked off by the gunshot, and a man in black hands the hat back
- Among the people who watch Marty confront Buford are a boy who is making a snack of corn on the cob, a man wearing a green hat and green vest, and the man's wife in a white dress.
- A bearded man listens as Marty accepts the challenge to a duel ("I do my killing after breakfast.")
- "Festival Man #1" (Steve McArthur, wearing a red shirt and black coat, tells Marty "I'm glad someone finally got the gumption to stand up to that son of a bitch!"
- "Festival Man #2" (John Ickes) adds, "You're all right in my book, Mr. Eastwood!"
September 6 (Sunday)
As Marty walks down the street on Sunday morning, everyone knows about "Clint Eastwood" and his impending duel with Buford Tannen.
- A bearded man (publicist Michael Klastorin) tips his hat and says, "Say, good morning, Mr. Eastwood!"
- A man wearing a bow tie gives Marty a cigar. He is Michael John Mills, a make up artist.
- A preacher, one of four people passing by in a surrey with the fringe on top, holds out a Bible and says, "Good luck tomorrow, Mr. Eastwood! We'll be prayin' for ya!" He is Kenny Myers, another makeup artist.
- Finally, a grim man (Marvin McIntyre) asks "Mr. Eastwood" if he's interested in "a suit for tomorrow". It turns out that he is the undertaker.
- The undertaker's assistant is busy preparing a tombstone, having already carved the words "Here Lies" on the stone in the photograph.
September 7 (Monday)
- When Doc, broken-hearted, goes into the saloon, he sees a full house:
- A prostitute, dressed in black, says "Come on, right over here" to a prospective customer in brown coat and hat, and they go upstairs.
- Another prostitute, in red, says "Good to see you. Sit down." to a customer.
- The barbed wire salesman and his assistant (Tim Konrad) are sitting next to Doc as Chester serves Doc with a bottle of whiskey. Chester's assistant, Joey, is played by Joey Newington.
When Marty finds Doc in the bar at 7:45, Doc takes a drink and falls, face first, into a table where two men sit — one with a long beard, the other with a green bandanna. Another man, holding a mug of beer and standing near the entrance, looks on. After Doc runs from the bar shouting, he almost bumps into another man, standing outside.
Among the people who talk to Marty, besides the three old men, are a man with an eyepatch ("You're a coward!") played by Brad McPeters, and a toothless, bearded man ("and you'll be branded a coward the rest of your days!"), played by Phinnaes D. The man with the red vest, who saw Marty at the Festival, slides his revolver down to Marty.
At the railroad station, Clara encounters the ticket agent (Rod Kuehne) and the train conductor (Leno Fletcher). When Clara pulls the cord to stop the train, the barbwire salesman catches her. A box falls on his head. As she leaves the stopped train, another man gets off the train to watch.
After Marty's confrontation with Buford, a fat man with a beard helps Buford out of the manure. A kid ("Here's your gun, Mister!") hands Marty the revolver. The kid is portrayed by Glenn Fox. Marty then hands the revolver to Seamus, and rides off toward the train with Doc. There, they hijack the engine from the engineer and his assistant (Larry Ingold).
October 27, 1985 (Sunday)
Upon Marty's return to 1985, three people watch him roll down the track at the railroad crossing: a woman in a station wagon, a man leaning out his window on one side, and another man watching from his car.
While Marty and Jennifer are stopped at the light at Hilldale, two people can be seen walking up the sidewalk. As Needles and his gang race away from Marty, the driver of the Rolls Royce — who had pressed charges in another timeline — runs a stop sign, but Marty and Jennifer aren't there to collide with him.
The railroad crossing is devoid of people when Marty and Jennifer return, and nobody witnesses their final meeting with Doc, Clara, and the boys.