I think he'd be an awful choice. The action sequences hes done in Chuck, are laughable at best, they're overly comical action scenes most of the time too.
A Superman actor has to have a rugged look, which currently Zachary does not, nor can i find a point where he ever has, this current picture shows just that(you wouldn't want this as Superman) -
http://www.imdb.com/media/rm2360771328/nm1157048
To me he doesn't have the look of someone who could put on a cape and wear pants over tights and appear like he could really kick the villains ass, without looking a complete fool.
Plus it would just end up looking like Chuck had showed up to a Halloween party(wearing a Superman costume) in December.(In other worRAB he'd look like a complete ass).
I think Zachary would make a great Flash though(as long as he played along the same lines as the current Smallville Incarnation), as there is a great comical presence to that character, which Zachary could pull off well.
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Article excerpt;
That is where the re-direct comes in, everyone will now start going through TV history of the past 5 -10 years looking for this popular TV actor, that no one would think of.
So if you think of it as a re-direct, then you think popular TV actor that everyone would think of, which would be Tom Welling, as hes played the role for 10 years, so he would be the most obvious choice.
Its been mentioned in the past that hes also been reluctant to wear the costume, hence why everyone would be thrilled if he did pull on the blue tights.
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Lastly as an aside, Michael Shanks would make a better Superman than Zachary Levi, as he has a much larger physique than him, and still carries a great comedic presence, but is also believable.