猛烈認真 Call Him Old Master Sun
他卻是個猛烈認真之士。汝等見了，須要行個大禮，叫他聲孫老爺，他就招架了；不然啊，他就變了嘴臉，這事卻弄不成也。」他却是个猛烈认真之士。汝等见了，须要行个大礼，叫他声孙老爷，他就招架了；不然啊，他就变了嘴脸，这事却弄不成也。」
逐句解 Line by line
"He is a fierce and earnest sort of man."
認真 in this period is nearer to 'takes things in earnest' than to the modern 'conscientious'. It is a warning rather than a compliment.
"When you see him you must make him a full bow and call him Old Master Sun, and then he will take you on;
行 is xíng, to perform — 行個大禮 is to make a full bow. 招架 is to take up an opponent's blow in a fight, so what Pilgrim will 'take on' is the business itself.
"but if not, he will change his face on you, and the thing will come to nothing."
The source cuts this from the clause before it at a semicolon, and the two stay apart. 不然啊 opens the alternative rather than continuing the parallel, and a half that turns against what precedes it is never joined to it.
This is Eight Rules coaching a crowd in how to flatter his brother, and every word of it is accurate. Pilgrim does respond to a full bow and the title; he does change his face when he is not given them.
The instruction will be obeyed to the letter five passages on, and the crowd will get exactly the reception Eight Rules promised them. He knows his brother completely, which is why being played by him stings.