Here's every member of the McFly clan played by Michael J. Fox in the Eric Stoltz was cast as Marty McFly instead.
Stoltz filmed as Marty for over a month before Zemeckis realized his performance just wasn't vibing with the film's comedic tone. The actor was fired and a deal was struck with Fox to film both Family Ties and Back To The Future at the same time, and a classic was born. The movie is still beloved decades after its original release, and it was enough of a success that two sequels were shot back to back and released in 1989 and 1990 respectively.
Back To The Future's timeline hopping allowed Fox the chance to not only play an older version of Marty but other of the McFly family too.
Seamus McFly
Back To The Future III - which creates a Doc plot hole - introduced Seamus and Maggie McFly, played by Fox and Lea Thompson. They're chronologically the oldest of the McFly family introduced in the trilogy, with the pair being Irish immigrants who move to the U.S. in the 1880s and have a child named William. Seamus runs into Marty - using the name "Clint Eastwood" - after he traveled back in time from 1955 in order to prevent Doc Brown's (Christopher Lloyd) death in 1888.
Seamus appears to be unconsciously aware of his family connection to Marty, and he gives "Clint" shelter, food and leads himino to Hill Valley. The character of Seamus was originally intended for Crispin Glover - who played Marty's father George in the original - but after the actor declined to return for either sequel, Fox was cast instead.
Marty McFly
Marty is introduced in the first Back To The Future movie as something of a "slacker" teenager who loves music, skateboarding and his girlfriend, Jennifer. Prior to the start of the story, he formed an unlikely friendship with local scientist Doc Brown. Marty is smart and self-aware for somebody his age, though his Achilles heel is being labeled a coward, which causes him to react impulsively or foolishly.
In the first entry, he's accidentally sent back to 1955 in Doc's DeLorean time machine. Not only does he have to find a way back to his timeline, but unite his parents as teenagers after interrupting their first meeting. Marty is often forced into awkward or outright dangerous situations throughout the trilogy, but his quick thinking usually saves him. Fox also plays Marty at age 47 in the year 2015, where part of his life was derailed following a car accident due to Marty being called "chicken" by a local bully, which forced him to give up music. By the end of Back To The Future III, Marty has shed the need to prove he's not "chicken" and declines that faithful race.
Marlene McFly
Marlene McFly is the teen daughter of Marty and Jennifer (Elizabeth Shue) and was introduced in Back To The Future Part II. Doc and Marty travel forward in time to stop her and her brother from making a mistake that would get them both sent to prison, but little is revealed about Marlene outside of her brief screentime, where she bears an uncanny resemblance to her father. She appears to be close to her grandparents and brother but is usually annoyed when her father asks a question.
Marty McFly, Jr
Marty, Jr is the final Back To The Future character played by Fox. Marty and Doc's actions prevent him from taking part in a robbery that would have ruined his and Marlene's lives, and he's depicted as both dim and an easy target for bullies. This makes him like his grandfather George when he was young, though the various timeline alterations in the sequels may have helped him become more assertive.
