Some characters are inseparable from the actors who play them. Marty McFly is Michael J. Fox, period. It's hard to believe that such iconic castings were sometimes the second choice. We're not saying there's an alternate universe where Hugh Jackman isn't Wolverine -- we're saying that this is the alternate universe.
In fact, this happens often enough that we're starting to believe that Fate intervenes to put the right performers in the right role. Just look at all these examples we found.