A few years ago a movie was done about a squad of replacement football players who played during an NFL strike.
Extending this phenomenon to acting-- there are certain actors which just seem to be lower budget versions of bigger name actors.
Big Budget: Charlize Theron
Lower Budget: Gretchen Mol (although, not as talented, she is equally HOT, if not HOTTER)
Big Budget: Jack Nicholson
Lower Budget: Michael Ironside (c'mon, please tell me I'm not the only one who sees this)
Big Budget: Johnny Depp
Lower Budget: Skeet Ulrich (why Ulrich isn't in prison for Grand Theft DNA is beyond me!)
Big Budget: River Phoenix
Lower Budget: Christian Slater (this one actually happened, Phoenix was initially cast as the interviewer in Interview With a Vampire, then of course, he died and had to be replaced)
Big Budget: Tom Cruise
Lower Budget: Stuart Townshend (I'm not saying I agree with this one, but Tom Cruise played L'Estat in Interview With a Vampire, Stuart Townshend played L'Estat in Queen of the Damned)
Big Budget: Matthew McConnaughey (sp?)
Lower Budget: Josh Lucas (it's that southern drawl and overall demeanor)
Big Budget: Keanu Reeves
Lower Budget: Jason Patric (Speed and Speed 2 need I say more?)
Big Budget: Jude Law
Lower Budget: Stephen Campbell Moore (see Bright Young Things, while Moore doesn't look much like Law, his mannerisms are quite similar and you can't help but get the vibe that he only got the role because they couldn't afford Jude Law and/or he wasn't available)
This has even spawned to lookalike stand-ins when you need foreign accents. As much as I enjoy John Cusack's acting, I must admit he couldn't pull off a British accent to save his life, and much to my relief, and the relief of his fans he's never tried. SOOOO, if you need John Cusack with a British accent you hire.... Michael McFadyen (see Pride & Prejudice you'll understand).
And then there are the actors that are interchangeable, they probably fall into the same price bracket so it doesn't matter which gets cast because the role is so vapid either/any could play it... The strongest examples of said phenomenon exists within the realm of romantic comedies:
Hugh Grant/Richard Gere/Tom Hanks
Julia Roberts/Meg Ryan
It's just a matter of time before we get a romantic comedy featuring Tom Hanks and Julia Roberts or Hugh Grant or Richard Gere with Meg Ryan!