"A different idea wanted to be written" oh yes, I know the feeling! But this one got its turn now and it's great.
I don't know if I've seen the Robert Downey Jnr one and I'll have to look out for it, a quick google suggest it is called "Hearts and Souls" :)
The one I was thinking of was "Ghost Town" with Ricky Gervais (I was on the fence about watching as I'm not always fond of his style of humour but I quite enjoyed the movie as an afternoon treat!)