My question was inspired by this question.
Suppose I see some "suspicious" activity on a New York City subway. I film this activity and report it to the police. The police decide on the basis of the film that someone was committing a robbery. They identify and arrest a suspect, identify and present a victim, etc.
I am introduced in court as the person who took the film. I take the stand and testify that I did so. I then disclaim knowledge of any actual crime, and claim only that the activity looked "suspicious," and I took it and handed the film to the police.
I would expect the defense to cross examine me about the film. "Where did you take this film? When did you take this film? At what angles/lighting, etc. did you take this film? How long have you had the camera? How much experience have you had operating this or similar cameras? How did you operate this camera?" etc.
But suppose the defense goes into other areas: "Do you feel that a crime has been committed? Why did you go to the police? Do you know if the defendant is the same person as in the film? How about the victim? (I did not introduce any of these topics on direct examination.")
What then happens if I say, "I don't know whether a crime has been committed? or "I don't know whether the defendant is the same person as in the film?." etc. and, "all I can say is that I took this film and the film speaks for itself." and "After receiving the film, the police acted independently of me, because I told them nothing else."
Is the defense allowed to question me on those grounds? Will my answers stand up if they are true?