Again, I don't think he wouldn't give it his all. From all appearances he seems to take his craft very seriously, so I don't think he'd "phone it in". My point is I want someone who is genuinely excited about playing Lex Luthor. I want someone who feels passion for the role and is excited to play it. Just because Phoenix is good at as job doesn't necessarily mean he'll like playing Lex Luthor.
I'm also not saying he can't be excited.
Totally understand what you're saying. I see it a little bit like red skull in the Captain America movie. Hugo did not seem to be excited about the role, but I thought he did a fairly good job. I personally think that Phoenix is the superior actor, so I think we would get one of the best villain performances in a DC/comics movie (rivaling the two jokers of course) and the worst a solid, well executed acting job.