Marius' character isn't developed much in the musical, but in the novel he is given a good amount of space. He is a man who has always been torn between loyalties. His early loyalty to his grandfather conflicted with the loyalty he owed to his father; his loyalty to his father later conflicted with his love for Cosette; and his love for Cosette conflicted with his devotion to Enjolras. With so many inner conflicts, it's easy to see how he might do something stupid every now and then.
The picture you see is of Peter Lockyer. The sound you will hear when you click on the picture is Michael Ball singing "Empty Chairs at Empty Tables," an emotional song if ever you heard one.
Empty chairs at empty tables, where my friends will sing no more. (Characters)