What would be grounds to prevent a person from testifying at a criminal trial?

A person may be prevented from testifying at a criminal trial if they are deemed incompetent to do so, such as being unable to understand the proceedings or communicate effectively. Additionally, testimonial privileges, such as spousal or attorney-client privilege, can bar certain individuals from testifying about specific information. If a witness poses a risk to themselves or others, or if their testimony is based on hearsay or other inadmissible evidence, they may also be excluded from testifying.