The Hunger Games star teamed up with top mentalist Keith Barry to learn the art of mind trickery for the film, in which he plays a top hypnotist, and decided to try out his new powers on his pal.
He recalls, "I started hypnotising Mark and Keith kinda took over... There's, like, a moment where the subconscious mind kinda rises up and the conscious mind drops and I was failing right at that moment, so he stepped in and he kept going and his thing was to convince him that everything was green. Mark just starts freaking out."
But Ruffalo is such a great actor, Harrelson fears his co-star was just pretending to be under their influence: "I feel pretty sure he was (acting) but I've never gotten to the bottom of it."
And the actor insists he won't be trying to put anyone else under his spell in the near future, admitting, "I haven't had much success."