Yes, I agree, communication is the key, but I would add attention to detail. From the onset, the engineer must ask the client what his objectives are, what he wants to achieve, what tones he's got in his head, if he has some CD's that he can let him listen to for reference purposes, etc. That way, the engineer can "engineer" the ideas and concepts into workable reality. But normally, when artists book a studio, they have certain "sounds" in their heads and they are in control. In that situation, the engineer just does what the client wants and should only "suggest" that the tone sucks or the arrangement of the song for that matter, if his opinion is sought. Then, he does what he, the engineer, thinks should be done to improve the sound while at the same time, asking the client what he thinks of what he is doing. Remember, the client should still be in control of the entire production In the case of an established client, the engineer must make his own research on how the client makes his records, how his tones sound, etc. That way, the engineer is prepared; there are no surprises and everyone just lives happily ever after.
FWIW