"For a celebrated actor, I was surprised at how ordinary he was." The "celebrated actor" refers, of course, to "he" (not "I"). However it seems to me the grammatical construction is wrong. By substituting the preposition "for" with "as" to read "As a celebrated actor, I was surprised at how ordinary he was," is now grammatically correct but drastically changes the meaning. How can I say what I want to say in plain conversational English?
For a celebrated actor, I was surprised at how ordinary he was.
This is a perfectly acceptable use of for indicating a comparison, and it's pretty clear that he is the actor (although in theory it could refer to the I, in practice the choice of for over as would leave no doubt in the mind of the audience). If you wanted to make it doubly clear, you could change the order:
I was surprised at how ordinary he was for a celebrated actor.
The way the sentence is written "for a celebrated actor" modifies "I", which is not the intended meaning. This is called a dangling modifier.
The following sentences avoid this ambiguity:
I was surprised that for a celebrated actor, he was pretty/rather/quite ordinary.
I was surprised that he was such an ordinary guy, despite being a celebrated actor.
In writing, I don't have a huge problem with your original formulation. I can't see a way to rearrange things that makes it clearer without making it more awkward.
For conversational English though, its way too long-winded. If I was just talking to my buds, it would be more like, "I was shocked. He's just a regular guy, not some hoity-toity1 celebrity."
1 - Sprinkle expletives to taste.