It's true that an Anna Faris-Andy Samberg body-switching comedy will probably not amount to very much, cinematically. But it's also worth remembering a premise as stupid as that and then some that became a terrifically entertaining movies in the right, er, hands:
It would be easy to say that All of Me works because Steve Martin and Lily Tomlin are brilliant. But the movie actually needs more explanation like that. The premise, that a swami is going to move the soul of a rich, crotchety terminally ill lady with no friends into the body of a sexy blonde employee who believes her soul is headed off cheerfully into the ether via a great deal of silliness, is absurd. It's the kind of thing that if Mike Myers did it would be about the ridiculous guru and would be terminally awful.
But All of Me works, and doesn't just work, is wonderful for a couple of external reasons. First, the silly premise is totally beside the point: it's just a vehicle for Martin and Tomlin to get to know each other, much as being forced to work together at a television station, or getting accidentally pregnant, are to today's romantic comedies. Second, the writing is often genius. It reads like Woody Allen at his best in short fiction. In what romantic comedy today, much less a body-switching one, would a lawyer declare to his rather difficult rich-lady client, "Just because my grandfather didn't rape the environment and exploit the workers doesn't make me a peasant. And it's not that he didn't want to rape the environment and exploit the workers, I'm sure he did. It's just that as a barber, he didn't have that much opportunity"?
But mostly the movie works because Martin and Tomlin are both just genius physical comedians. You believe Tomlin's in there somewhere, because Martin's body is behaving so bizarrely.
I am entirely sure that Samberg and Faris won't be the beneficiaries of such good writing. And I'm less sure, though still reasonably confident, that neither of them can quite pull off the physical comedy. Faris wasn't bad, teetering on absurd heels in The House Bunny, or miming sexy-Marilyn-over-a-grate-gone-horribly-wrong-and-resulting-in-burns. But Martin lives in another land of comedy altogether, beyond the anarchic, into the impossible. I don't think Samberg has the chops at all. My bet: lots of groping at newly-discovered body parts and funny walks. And a distinct lack of entertainment.