I'll have to watch Paint Your Wagon
again sometime. I don't remember Lee Marvin singing in it, and it might be a blocked bad memory. I'm trying to imagine hearing him sing, and it ain't pretty.
To me it tends to be highly subjective and a case-by-case issue, but I like to think I'm pretty open to someone's attempt(s) to croos over to another genre.
One cross over that repeatedly amuses me are hard core opera singers going pop, and vise versa. Listening to Sting(and I like him, mind you) sing Nessun Dorma
is frightening, much more so in the case of Michael Bolton.
And then there's Pavarotti singing Maria