suspend (one's) disbelief
To ignore or look past one's incredulity at the improbability of some piece of fiction, especially a film or video game, in order to better enjoy the experience of the work. I'm more than willing to suspend my disbelief a little for a cheesy action film. I find that I can never really suspend my disbelief when I watch a movie that's crammed full of CGI.
You couldn't (do that)!
and You wouldn't (do that)!an indication of disbelief that someone might do something. Bill: I'm going to run away from home! Jane: You couldn't! Bill: I get so mad at my brother, I could just strangle him. Tom: You couldn't do that!
suspend disbelief
To accept as plausible something one knows to be untrue, especially the setting and plot of a drama or fiction so as to allow the appreciation of art.