Frank is a laid back kind of guy. He can be too calm and quiet sometimes. According to PKM, he should have low, calm and relaxing tones. I interpret Frank as kind of a deadpan but still fatherly and at least, lack of a better word, smooth? He may be flat, but not too flat, there's still gotta be expression and emotion in him, at least. Then again, my opinion, up to anyone's interpretation.