Zhang Fei's character development, plus his character flaws (alcoholism, being an ass, etc.) makes him my favorite character in the novel bar none.
He shows his cruel side by whipping and killing a lot of people (the inspector, his soldiers, etc.), his alcoholism by losing Xuzhou, his genius by his tactics (especially using his reputation for being an alcoholic), his compassion by sparing Yan Yan, and his loyalty when he charged Guan Yu, thinking that the latter betrayed Liu Bei. He also shows his comedic/childish side, during the 3 visits, or when he threw a fit when he Zhao Yun was chosen to lead a campaign over him.
Plus, he held not only military positions, but some of the highest ranking administrative positions in the kingdom. He was grand administrator of several domains, including Hanzhong, and he proved his intellectual judgement by humbling himself before Pang Tong when he realized how talented the latter was. He may have been a brute, but he was far from mindless, even in the novel (at least in the later parts).
I can also think of many individual scenes where he not only illuminates his own personality, but those around him. During Liu Bei's battle against Wang Zhong and Liu Dai, he was hesitant to use Zhang Fei to engage Liu Dai, for fear of his brother killing their opponent. When Zhang Fei captured Liu Dai alive, he rode back to his brother, saying "You said I was too rough. How now?" Which Liu Bei replied, "If I had not put you on your mettle, you would not have evolved this stratagem." And Zhang Fei laughed. Even though the dialogue is very short, you can almost see the brothers smiling, very pleased with themselves, throwing good-natured jabs at each other.
Another one is Chang Ban, the dialogue between Zhao Yun and Zhang Fei.
Chapter 41 wrote:Zhao Zilong shouted and rode at the captor who was speedily slain by a spear thrust and his captive was set free. Then taking two of the horses, Zhao Zilong set Lady Gan on one and Mi Zhu took the other. They rode away toward Long Slope Bridge. But there, standing grim on the bridge, was Zhang Fei. As soon as he saw Zhao Zilong, he called out, "Zhao Zilong, why have you betrayed our lord?" "I fell behind because I was seeking the ladies and our child lord," said Zhao Zilong. "What do you mean by talking of betrayal?" "If it had not been that Jian Yong arrived before you and told me the story, I should hardly have spared you." "Where is the master?" said Zhao Zilong. "Not far away, in front there," said Zhang Fei. "Conduct Lady Gan to him. I am going to look for Lady Mi," said Zhao Zilong to his companion, and he turned back along the road by which he had come.
Chapter 42 wrote:But in his rear arose another tumultuous shouting, seeming to rend the very sky, and Wen Ping came up behind. However, although the man was weary and his steed spent, Zhao Zilong got close to the bridge where he saw standing, all ready for any fray, Zhang Fei. "Help me, Zhang Fei!" he cried and crossed the bridge. "Hasten!" cried Zhang Fei, "I will keep back the pursuers!"
Another short dialogue, but to me it really shows how the top generals acted toward each other. Here we see Zhang Fei being gruff and suspicious of Zhao Yun, but the way they refer to Liu Bei as "the master", and the way that Zhang Fei responds to Zhao Yun's desperate cry for help, it all shows that while these men were powerful forces unto themselves, they were still loyal and subservient to "the master".
He is definitely one of, if not the most colorful character in the novel.
I like you...but you're crazy.