It isn't murky, because law is about intent more than result. It doesn't matter if they hired someone who sounds like Scarlett, it matters if they intended to do so.
If they accidentally hired someone who sounds identical, that's not illegal. But if they intended to, even if it is a pretty poor imitation, it would be illegal because the intent to do it was there.
A court of law would be looking for things like emails about what sort of actress they were looking for, how they described that requirement, how they evaluated the candidate and selected her, and of course, how the CEO announced it alongside a movie title Scarlett starred in.
Under what legal theory is intending to do something which is legal (hiring a person that has a voice you want) becomes illegal because there is another person who has a similar voice?
It's not intending to do something legal, it's intending to do something illegal: Stealing their likeness. The fact you used an otherwise legal procedure to do the illegal activity doesn't make it less illegal.
How can something be illegal if every step towards the objective is legal?
This would result in an incoherent legal system where selective prosecution/corruption is trivial.
What's illegal, in general, is not the action itself but the intent to do an action and the steps taken in furtherance of that intent.
Hiring someone with a voice you want isn't illegal; hiring someone with a voice you want because it is similar to a voice that someone expressly denied you permission to use is illegal.
Actually, it's so foundational to the common law legal system that there's a specialized Latin term to represent the concept: mens rea (literally 'guilty mind').
It is legal to buy a gun, and legal to fire a gun, and it can even be legal to fire a gun at someone who is threatening to kill you in the moment, but if you fire a gun at someone with the intention of killing someone that happens to be very, very illegal.
So? You're merely (correctly) pointing out that the acts have consequences that are of wildly differing severity. Not that one is a legal and the other is not.
