Under what circumstances might it be advisable for someone in the firm other than you to handle an interview with a client or witness? Can a client restrict the use of information obtained as part of...

Under what circumstances might it be advisable for someone in the firm other than you to handle an interview with a client or witness?

Can a client restrict the use of information obtained as part of the investigation in preparation for trial even if doing so will have adverse consequences, in the opinion of the attorney? Why or why not?