Titus Pullo, like so many ESTP's, is the team's superstar (their best fighter in this case), and believes that therefore he can do whatever he wants, screw regulations. Vorenus, in a very ISTJ drill sergeant like fashion, corrals him into line.
(This is like the story of every sports team you were ever on in high school. Some ESTP is the top scorer and also the rule-breaking hotshot. How does the coach react?)
It's a very nice—if stereotypical—scene about an ESTP trying to slip authority and an ISTJ reeling him in.
Later, they become friends. Pullo's influence softens Vorenus—at least a bit—and Pullo becomes more responsible. Rome is kind of a two season long buddy movie about an ESTP and an ISTJ.