I'm glad that you brought up Charlie Sheen, because he's a great example of what I'm talking about. Had the writers of Two and a Half Men kept the same concept in their writing, the smart-ass one liner type lines directed at Alan and the kid, and replaced Sheen with any one of a number of actors it probably would have worked.
As a guy on the writing end of a two-bit production or two (stage not TV, no, didn't make a ton of money,) I'd love to say you are absolutely right and buy right into the "the writing is the whole gig" theory.
But I think the direction and acting is like a collaboration w/the writer... of course, the way we do it in the U.S. seems to favor the director as the
artiste, not the scriptwriter.
I will say though, you can't really make 2 1/2 men worth without Sheen... I think it's because the joke has always worked because the self-absorbed jackass Charlie really
is the self-absorbed jackass Sheen. You could maybe plug in Emilio Estevez, actually call him Emilio, and ask why they have different last names if they're brothers etc. etc. etc. ... have some fun with the Mom explaining the half-brother thing & her various marriages... actually I could see that working.
But you couldn't just plug in anybody. I think they picked a fairly one-note guy, who always plays the same type, to replace the Sheen type with the Kutcher type.... that's pretty close to the show formula, but it totally doesn't work. The premise of why he's there and still letting Allen stay there doesn't work. The show without its usual meanness doesn't work. Hell, even the kid growing up screws up the show somewhat. It's like "Hey kid, you're 22, are you still in 5th grade?"
RIP Charlie Harper.