Clients are sometimes concerned that they will lose control in hypnosis. 

However, general consensus indicates that regardless of how deeply people may go into hypnosis and however passive they may appear to be, they actually remain in full control of the situation. 

They are fully able to talk if they wish to or not as the case maybe.  It is likely that the notion of a loss of control stems from most people's misconception of stage hypnosis, wherein participants are apparently persuaded to perform all manner of foolish acts. However you should be aware that participation in a stage act is an entirely voluntary process (thus "permission" is already given to the hypnotist) and that there can be no such volunteer who is unaware of exactly what they are letting themselves in for!

** Information provided by the GHR , the hynotherapy regulating body.