Every juror knows the defendant is the most biased person in the courtroom. The prosecution is not allowed to say so, but everyone knows it. Criminal defendants are generally not very bright. They can easily be tripped up or impeached by the prosecution. They also have a unique way of irritating a jury.
Surprise is the greatest weapon of the accused.
But if one feels compelled to answer the prosector's opening statement, keep it very simple. The defense will show you that Mr. Jones acted in self-defense. Please listen to the evidence with this in mind, so that Mr. Jones will receive the fair trial that you promised him. Getting too specific can lead to the defense admitting facts that the prosecution is actually unable to prove. On the other hand, promising facts that the defense intends to prove can be disastrous if one is unable to do so.