Let's Say You Are a Defense Attorney and Are Defending

Let's say you are a defense attorney and are defending a client on a murder charge. The victim was found in a dark alley. A witness had a glimpse of a shadowy figure standing over the body. The police thoroughly interrogate the witness and arrest a young man. The man has an alibi. There is no other evidence against him, except the testimony of the witness. The witness is positive. What factors may lead you to doubt this strong testimony of the witness? Pretend you are the defense attorney and, using your knowledge of the problems with eyewitness testimony and memory, tell the jurors why they should be doubtful of the witness.