Case study # 1: Lindsay Lohan
Hey I'm not going to lie -- I once signed an online petition "make Lindsay eat." Would that anorexia were our only concern with this one! Alas after being blessed with remarkable opportunities, luck, looks, and talent, Lindsay altered her trajectory from the next big star to the next big reality show star through drugs and crime.
Now while she was still relatively successful, movie producers wanted someone in her same vein, but much much cheaper. And thus was Emma Stone discovered.
Cast for an unremarkable role in a surprisingly tender, enjoyable movie ("Superbad") -- a role that would have been beneath Lohan -- Stone parleyed her performance here into progressively larger roles until she has now been tapped as Gwen Stacy in the second Spider-man trilogy. (Note: they have yet again wasted a perfectly good redhead to play this blond character.)
Emma looks like Lindsay ... and she now has her career.
Case study # 2: Michael Cera
Pity poor Michael Cera -- such a lovable character actor, but, man, he cannot perform at the box office. While he's been very prolific, he has not proven he can lead a film: Year One, Youth in Revolt, Nick and Norah's Infinite Playlist, Paper Heart and Scott Pilgrim vs. the World have all been duds, even though Pilgrim was still a great film.
Ironically, he's still been able to ask enough of a salary that tight-pursed producers started turning to Michael Cera look-alike Jesse Eisenberg for Adventurelandand Zombieland in roles that were obviously written with Cera in mind.
When Zombieland (which was a clever idea but still only an OK film) made over $100M, Eisenberg was tapped to headline The Social Network, which has been doing well enough that Eisenberg has got to be feeling pretty good about his future.
Now of course Michael Cera would have been able to play the role, and it would have been nice to see him play a character other than George Michael Bluth.
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Just because someone gets there first doesn't mean they'll be best. Frequently the first generation of a product gets improved by the person watching their mistakes. The examples are copious: Goldwater/Reagan, Gobots/Transformers, Old Testament/New Testament, and (germane for this post) Myspace/Facebook.
But it isn't terrible when one of your favorite stars drops a bit, because that means they might take boutique roles they'd overlook otherwise. Though at this rate, maybe Jesse Eisenberg will play George Michael in the Arrested Development movie, and they'll get Emma Stone to play Maebe Fuenke.