Screen Junkies remember Eric Stoltz as Marty McFly

From an article: "8 Classic Characters That Were Almost Played By Someone Else" Screen Junkies mention the original selection of Tom Selleck as Indiana Jones

Marty McFly: Unlike many actors on this list, Michael J. Fox was the initial choice for the role that made him famous. Conflicts with his “Family Ties” shooting schedule nearly kept Fox out of “Back to the Future” and, for a time, he was replaced by Eric Stoltz. Fox returned to the role when Stoltz showed he wasn’t right for the part of a time traveling hero with his uber-serious approach. Producers worked with Fox’s TV schedule and the rest is history.

http://www.screenjunkies.com/movies/movie-lists/8-classic-characters-that-were-almost-played-by-someone-else/3/

Check out the Future Pedia page for Eric Stoltz

Eric Stoltz is an American actor who was originally cast for the role of Marty McFly when production of Back to the Future began on November 26, 1984.

Among the scenes that were originally filmed with Stoltz were:

(with Christopher Lloyd) Doc's first time-travel experiment at the Twin Pines Mall
(with Crispin Glover, Tom Wilson, and others) Marty's first encounter with George, Biff, and Biff's gang at Lou's Cafe
(with Glover) The scene where Marty is hit by Sam Baines's car (in commentary with Bob Gale notes that this was the first scene shot when filming began).
(with Lea Thompson) Marty's first 1955 meeting with Lorraine (a still photo is featured on the DVD bonus feature, "The Making of the Trilogy")
(with Lloyd) The mind-reading scene
(with Lloyd) The 1.21 gigawatt conversation between Marty and Doc
(with Glover) Marty encouraging George to go into Lou's Cafe to ask out Lorraine. Parts of the scene were filmed from inside the cafe, with Lorraine, Betty and Babs in the foreground, and Marty and George seen through the window, a perspective not used when the scene was refilmed.
(with Glover) Marty describing to George the plan for the fake fight.
(with Lloyd) Marty and Doc arguing over the "Do Not Open Until 1985" note


In addition, a scene filmed with Stoltz, but not re-filmed with Fox, had Marty being held in detention with Mr. Strickland and escaping by setting off a fire alarm in an elaborate manner. The fourth draft of the screenplay, completed a month before shooting began, provided the script originally written for the scenes filmed with Stoltz. Besides the detention scene, Stoltz's Marty did not wear the down vest that would be mistaken for a life preserver, and drew attention instead for having green tennis shoes.

http://backtothefuture.wikia.com/wiki/Eric_Stoltz