The past is both a wonderful and horrific thing for the characters in Selznick's The Invention of Hugo Cabret and Scorsese's Hugo. Hugo Cabret himself, in both versions, enjoys remembering his past with his father - they were happy inventors who did things together and generally enjoyed their time together, making them happy memories. Rebuilding the automaton, continuing his father's work, is Hugo's way of reliving the past and pushing away the fact that he is an orphan; that Uncle Claude is unloving (and has been missing for some time); and that he is all alone.
Georges Melies' past, on the other hand, is an unhappy one; his former glory as a famous filmmaker destroyed by the horrors of World War I and financial destitution, he wishes to forget the past as much as possible. Since he feels as though he has outlived his usefulness - a "broken windup toy," as he calls himself in the film - remembering the past reminds him of what he has lost, and he hates that. When Hugo accidentally reveals the old storyboards and drawings of his inventions and special effects for his old films, he rips them up in a rage, responding with anger at the past. Over the course of both the book and the film, Hugo's quest is to get him to remember the past the way he does his own: fondly.
The way in which Hugo gets Melies to think more fondly of the past - to "fix" him - is to get him to think about the past the way his audience (at the time and in the film/book) does. Melies, due to his experiences, believes that no one cares about his films. However, the emergence of Rene Tabard, the author of The Invention of Dreams, demonstrates that there is a whole society of film buffs - cinephiles - who believe in Melies' work and are amazed by it. Melies simply refuses to believe that he has an audience, making him hate the past and resent it for being a time when he had a purpose, and someone to show his films to. Melies' trigger for remembering the past is being shown the automaton - his one prized possession. Thinking that he had lost all of the physical things that made him who he was, seeing the automaton and the rescued print of A Trip to the Moon shows him that his past, and the audience's love for him, are still around. This gives him renewed faith in his work, and allows him to "work again."
The importance of remembering the past is shown to be a recognition of who you are, and the lives you have touched. Hugo remembers his father, who was a big influence in his life, and Melies finds renewed faith in the audience that fondly remembers his work. The audience gives validation to that remembrance, by loving his films and showing him that love; Melies, recognizing the joy that having his films seen and appreciated brings to him, soon remembers the past - the time when he was useful, and brought joy to the world - fondly.
