Claude Rains portrays the district attorney who takes the case, and wants to make it big important conviction for himself to further his political career. Rains although in a way portrays the villain in that Griffin is always very concerned about his career rather than seeking justice. Although to be entirely fair to his character the evidence does not suggest that the professor was obviously innocent or obviously guilty. The sentiment that is created by the atmosphere within the town though makes it clear that innocent or guilty the professor will not really be given a fair trial.
Claude Rains as usual is efficient in the role as the attorney making it clear that Griffin will win the case, and meet his ambition no matter what. Rains at first shows Griffin more than anything is being professional as he handles various town people who want him to do certain things. Rains shows a constant assurance that no matter what Griffin will be doing things his way, even if it does meet someone else's end, it is all still is all part of his own ambition. Rains makes it obvious that Griffin is a powerful force that will see the man found guilty through his skill as an attorney.
A great deal of screen time comes in when he is handling the courtroom scenes. Rains again is effective here as he shows the statesman aspect of Griffin leaves as he purposefully becomes a firebrand to keep passions high to ensure conviction. Rains is energetic here as he portrays the fierce nature of Griffin as an attorney as he cuts to the throat every moment in court. Rains is strong here although I would say that he is never quite as amazing in this way as he was in similar moments in Mr. Smith Goes to Washington.
I have to say that Rains does his very best but unfortunately the film sort of lets his portrayal down. Rains really wants to make the character complex more than the film wants to. Rains tries his best to show Andy Griffin as a man with an ambition but not an evil man, the film pretty much does want him to be a one dimensional villain. Rains deserves credit though for trying to convey some sort of internal struggle at the end in Griffin over his actions, but the film frankly does not care as it suggest through the script that Griffin does not really care in the end.
Claude Rains really has to deal with a character that is less complex than he should be considering Rains takes every attempt to bring more to the character, but the film never allows him to go off on any of these complexities he tries to bring to life. Due to this problem Rains does not give a performance that is in line with his later greater work. Rains though still does try to make the most out of his performance, and gives an fairly effective performance even if it is not all that memorable of one.