Not sure I completely agree with AS on the idea your protagonist should be 'likeable'. Maybe in Pixar's universe s/he should be likeable but in the bigger screenwriting universe, they should just hold your interest, i.e. be 'compelling'. There are dozens of examples of people in thrillers and comedies who you would never introduce to your mother, but who's every move has you gripped. From Willie in Bad Santa (2003) to Dan and Alex in Fatal Attraction (1987), the hits keep on rolling. Sure, if you movie involves getting the audience to root for the Protagonist then likeable is a must. But alternative approaches are available. I expect each movie genre has it's own 'rules'. Pixar still rocks!