While Tom Hardy may have been responsible for the voice, the actor tells Entertainment Weekly that the sound team was actually responsible for the character, because they made sure the voice was cued properly so that Hardy could play against it accurately.
While most actors these days are used to performing against nothing but a tennis ball on a stick to stand in for some CGI creature to be created later, while Venom could not be seen, he could be heard on the set of Venom as the actor helped create the voice, which was then played back by the actors so that everybody, including Hardy himself, could react to it while Hardy was performing his lines as Eddie Brock.