( CNN ) -- Trial lawyers use an expression -- `` You ca n't unring a bell '' -- after a jury is exposed to something damaging . Even if a judge commands people to disregard that something , the harm is done once the words have been uttered . George Zimmerman may be wishing that several ringing bells sounded in his attorney 's opening statement last week could be unrung , beginning with a now infamous joke . After a streamlined prosecution opening in which a rapt courtroom was told of efforts to resuscitate Trayvon Martin 's lifeless form and heard a straightforward argument that Zimmerman shot Martin `` because he wanted to , not because he had to , '' Zimmerman 's attorney Don West chose to open his defense by telling a joke . Resorting to humor -- even in traffic court -- is fraught with risk , but seems a particularly unwise idea at the outset of a murder case in which the defense must counter the appearance of casual and callous indifference on Zimmerman 's part . Even worse , perhaps , it runs the risk of appearing to trivialize the trial itself . ( The joke chosen even implies that selected jurors who were n't savvy enough to dodge their civic duty will now have their time wasted . ) Prosecutors will tell you that the most significant challenge to securing a guilty verdict is to persuade initially resistant jurors to grasp the gravity of a case , making them at least open to returning a guilty verdict . Several post-joke comments made by West in his opening could unwittingly pave a path to conviction . West said that it was a dangerous dog in Zimmerman 's neighborhood that forced his hand , necessitating that he buy a 9 mm Kel-Tec PF automatic firearm . The attorney matter-of-factly suggested it was the American way for an ordinary person , faced with an uncontrolled animal in his community , to procure a firearm . ( Firing shots at a menacing dog could easily imperil others , of course . ) Arguing that during the confrontation with Martin , Zimmerman 's head was struck against the pavement , West told the jury : `` When you get your bell rung , stuff happens . '' While defending an emotional case like this is admittedly a tricky high-wire act , these words suggest an act of vengeance by an armed man rather than a genuine effort to avoid death . It will not be surprising to hear those words repeated back in the prosecution 's closing statement . Five of the six jurors are mothers for whom `` stuff happens '' may seem an insufficient accounting for the termination of a life cut short shy of two decades . By his lawyer 's account , Zimmerman 's worries were not confined to perceived community dangers , but to his own physical well-being . He wanted to learn martial arts , West said , but washed out of fight school , where he was described as too `` soft . '' Many defense attorneys deliver short openings or do n't open at all because of the risk of a bad backfire . West 's lengthy opening resulted in a composite picture of a person who easily conjures threats and fears , sought a gun as a leveler for his insecurities and whose response to these perceived threats could be seen as questionable and retaliatory . Criminal defendants are not only at the mercy of the state 's awesome powers , but also must live with the tactical decisions of their attorney . Trying to persuade an appeals court to overturn your conviction because of detrimental choices made by your lawyers at trial is all but impossible . On Monday , the prosecution resorted to the often potent technique of using a defendant 's inconsistent statements in an attempt to collapse his believability and blunt possible jury empathy for him . Prosecutors showed a video of Zimmerman giving an expansive explanation of what happened during a scene walk-through the day after the shooting , followed by playing a lengthy taped interview that was conducted by Sanford police a couple of days later . The prosecution hopes that jurors will agree that the more Zimmerman explains his actions , the more self-serving he sounds , and the less trustworthy he will appear , something vitally important for someone who , when all is said and done , offers the only account detailed enough to justify using lethal force . The opinions expressed in this commentary are solely those of Eugene O'Donnell .
Eugene O'Donnell : In George Zimmerman 's trial , his attorney began with a bad joke . O'Donnell : Resorting to humor in a trial is fraught with risk of appearing too casual . He says the defense must counter the appearance of callous indifference . O'Donnell : The more self-serving Zimmerman sounds , the less trustworthy he appears .