You can't duct tape anyone's mouth shut for more than a moment - small movements of the jaw and lips will loosen it. Despite this fact, Richard Haibach has been bound to a chair for hours without managing to loosen the small piece of duct tape that barely covers his mouth.
(at around 13 mins) When Patrick Jane is dictating and Ruby Hightower is typing, she types the word "love" before Patrick says it.