What makes a convincing protagonist? A few of my thoughts.

Protagonists are essentially at the core of a story. They are a force that helps to drive the tale onward. This is the person (or persons) whom the reader will journey through the story with and therefore it is essential that the protagonist is a many layered, intriuging character the reader can relate and empathise with. A boring character with weak dialogue and an uninteresting personality will make the character forgettable and bore the reader.
A backstory is essential - the author should know intricate details about their character or characters, that may not be revealed in the story at all. If the author knows these people they are writing about inside out, I think this shows in their writing.
Protagonists are the good guys, the steadfast heroes who never give up and fight for what is right. The reader roots for the protagonist to succeed and in first person or omniscent narratives especially, sees the world outlined in the tale through their eyes.They may be the reluctant hero, forced into the role through unforseen circumstance, or perhaps they made a mistake in the past and are doing all they can to right a wrong. A protagonist needs something that has driven them to be the way they are - there has to be reasoning behind their actions which will in turn give the story and world they inhabit an essential realism and make those twists and turns in the plot and the thrill of the action all the more engaging.