“You can’t be an asshole. There is not a really great trial lawyer alive who is a jerk, at least in the courtroom.”
It is your job to get the jury to like and trust you. You want the jury to not want to disappoint you. And to do that, you can’t be an asshole.
The only time you can act up is when the jury gives you permission to; for example, when cross-examining a witness who is clearly lying. But you must not overdo it. Because if you do, you run the risk of the jury feeling sorry for the witness.