How confident a witness sounds or acts may play a large role in a court case. When the witness picks out the alleged perpetrator, if they seem very sure that they have the right person, it's easy to assume that they do -- even when that person denies it with every breath.
The problem is that even confident people simply get it wrong. They make mistakes.
It's not intentional. Witnesses are not always misidentifying people maliciously. They simply think they are right when they're not. That belief that they're being honest is what gives them confidence. It does not matter if the belief is grounded in reality or not.
For instance, one woman was assaulted by a man and then confidently picked him out of a lineup for the police. She was sure, honestly sure, that he was the right person. The jury convicted him and he went to prison. He got a life sentence.
He served about 10 years of that sentence before DNA evidence came into the picture and cleared him. He'd never done it. The right man was identified and the innocent man was released.
The fact that she was wrong had not changed the way that the woman acted. She'd just made a mistake. She still felt very confident at the time and did not know about the error until that man had lost a decade of his life to a jail sentence he did not deserve.
That's just one example, but it helps to illustrate some of the potential problems we face with the modern criminal justice system. That's why it's so important for those accused of serious crimes to know all of their defense options.