That guy in the drama club who tells everyone he hates organized sports one day and then joins the football team the next? He's being duplicitous, or pretending to have feelings that his actions contradict.
In duplicitous, you can see the word duplicity, which means "doubleness." Someone who is duplicitous is almost like two people, saying one thing but then doing something very different, even contradictory. Someone who is duplicitous can also be called "two-faced," a vivid way to remember that this person shouldn't be trusted or taken at face value.