牛王模樣 In the Likeness of the Bull King
好大聖，即現本像，將金睛獸解了韁繩，撲一把，跨上雕鞍，徑直騎出水底。到於潭外，將身變作牛王模樣。打著獸，縱著雲，不多時，已至翠雲山芭蕉洞口。叫聲：「開門。」好大圣，即现本像，将金睛兽解了缰绳，扑一把，跨上雕鞍，径直骑出水底。到于潭外，将身变作牛王模样。打着兽，纵着云，不多时，已至翠云山芭蕉洞口。叫声：「开门。」
逐句解 Line by line
Excellent Great Sage! He resumed his own form at once, loosed the halter of the golden-eyed beast, gave a spring, mounted the carved saddle and rode straight up out of the bottom of the pool.
將 is jiāng, fronting the object. 解 is jiě, to untie. 撲一把 is a colloquial phrase for one vigorous movement — a spring, a lunge — and 一 takes yì before the dipping tone of 把.
Once outside the pool he changed his body into the likeness of the Bull King.
模樣 takes mú. The order of operations is careful: he keeps his own shape to handle the beast, and only puts on the other face once he is clear of the water and out of sight of the palace.
Driving the beast and riding the cloud, in no long time he was at the mouth of the Banana-Leaf Cave on Kingfisher Cloud Mountain.
Both instances of 著 here are the continuous zhe, driving and riding. The same character is the causative zhuó in the next passage, when he orders the maids about.
He called out: "Open the door."
Four characters, and the entire imposture. He does not announce himself, offer a story or prepare a manner. He gives an order at a door, in the voice of the man who owns it.
Two characters do the whole deception: 開門, open the door. No announcement, no cover story, no performance. He rides the husband's mount to the husband's door and gives the husband's order, and the door opens. The economy is the point — the disguise is not an act but a set of circumstances arranged so that no act is needed.
The sequence of the transformation repays attention. He resumes his own shape to deal with the halter and the saddle, because a crab cannot ride, and takes the Bull King's face only after clearing the water. The novel is consistently practical about its magic: shapes are tools, picked for the job in hand and put down when the job changes.