Who was originally cast for back to the future?
Michael J. Fox was the first choice to play Marty McFly, but he was committed to the show "Family Ties". "Family Ties" producer Gary David Goldberg felt that Fox was essential to the show's success. With co-star Meredith Baxter on maternity leave, he refused to allow Fox time off to work on a film. "Back to the Future" was originally scheduled for a May 1985 release, and it was late 1984 when it was learned that Fox would be unable to star in the film. Zemeckis' next two choices were C. Thomas Howell and Eric Stoltz. Stoltz impressed the producers enough with his earlier portrayal of Roy L. Dennis in "Mask" (which had yet to be released) that they selected him to play Marty McFly. Because of the difficult casting process, the start date was pushed back twice. John Cusack was also considered for the role. Johnny Depp also auditioned for the role of Marty McFly.
Who was originally cast as Marty McFly for Back to the Future?
Eric Stoltz
Back to the Future
No context provided
Who was originally cast as Biff Tannen for Back to the Future?
J.J. Cohen
Melora Hardin was originally cast in the role of Marty's girlfriend Jennifer, but was let go after Stoltz was dismissed, with the explanation that the actress was now too tall to be playing against Fox. Hardin was dismissed before she had a chance to shoot a single scene and was replaced with Claudia Wells. Actress Jill Schoelen had also been considered to play Marty's girlfriend.
Who was originally cast as Marty's girlfriend Jennifer for Back to the Future?
Melora Hardin
Back to the Future
Eric Stoltz
https://en.wikipedia.org/wiki/Eric%20Stoltz
Back to the Future
https://en.wikipedia.org/wiki/Back%20to%20the%20Future
Back to the Future is a 1985 American science fiction film directed by Robert Zemeckis. 
Back to the Future
 Shortly after principal photography began in November 1984, Zemeckis determined Stoltz was not right for the part and made the concessions necessary to hire Fox.
Back to the Future
Deluise, Zane, Tim Robbins, and J. J. Cohen were considered to play Biff Tannen.[6][44][45] Cohen was not considered intimidating enough against Stoltz, and the role went to Thomas F. Wilson; his first feature starring role.
Back to the Future
During production, the 1985 American science fiction film Back to the Future underwent a series of casting replacements. Eric Stoltz was originally cast for the role of Marty McFly but was later replaced by Michael J. Fox after Robert Zemeckis determined that Stoltz was not right for the part. Melora Hardin, hired to play the role of Marty's girlfriend Jennifer, was let go after Stoltz was dismissed, with the explanation that the actress was now too tall to be playing against Fox. Another change came with the character of Biff Tannen, who was going to be portrayed by J.J. Cohen but was ultimately not considered intimidating enough against Stoltz, and the role went to Thomas F. Wilson.