When we talk about a film's protagonist, we are not just talking about another character, but the central player ( or players if there are two protagonists such as in Romeo and Juliette) and is the reason for the story of the film to proceed. A strong film always has a strong protagonist.
So what makes a good protagonist? I think that we as an audience should identify with the protagonist and if we do, then we might actually care to see what happens next . What happens next , leads me to the single most important attribute about the protagonist and that is that they have a quest, goal, purpose, passion or mission! They want something, and its clear from the outset what they want. However, its going to be a monumental struggle to get it! The more difficult it is, the more interesting the story. Therefore if the character is flawed, there is already great difficulties to overcome. I think human foibles in the protagonist probably the single most important attribute in making me relate to them. Sometimes I may not even like the character, but as soon as I see that they have weakness, them I am willing to forgive them.
The protagonist does not have to be "good" or "bad" but they must be interesting! They can even be deliberately bad or evil ( which already makes them interesting). Here is where "contradiction" within the character arouses our curiosity about them. The more extreme the contradictions the more colorful. I think about Walter White in "Breaking Bad" who to me is one of the more fascinating protagonists. He is essentially a middle class chemistry teacher who lives in a typical suburban home and is married with two children the eldest son about 15 to 16 with cerebral palsy. Yet he has a quest to make money as a "Cook" of Crystal Meth dealing with extreme underworld figures including Mexican drug cartels and outsmarts them. A fine example of extreme contradictions between his conservative background yet criminal quest.
Another way to make them interesting is to make them humorous. If the story dictates that they be serious in order to build tension, then it can be very difficult . However that also provides the opportunity for some dry humour , so long as it breaks tension at the right time.
The final aspect that is needed by the protagonist, is a way to overcome the obstacles. I have a friend who wrote and illustrated for Marvel comics and is convinced that all protagonists have to possess a supernatural power. I am not sure that's true but I do agree that certainly gives them means to overcome their difficulties. I believe that circumstances, chance events , or maybe just simple persistence by the protagonist achieve the same outcome.
