"Much the same occurs when Lucien doodles a self-portrait in a sketchbook; without ado, the figure enters the film—a looming, crooked alter ego, played by an actor with a puppet’s mask. He continues to materialize, unsummoned, throughout Gainsbourg’s life, sitting down beside him to play a piano duet, or—once Lucien has grown up and changed his name to Serge—caressing, with elongated claws, the naked, snoozing body of one of his lovers. This peculiar being is never wholly explained. He remains a benign Nosferatu, halfway between the demon that dogs romantic souls, luring them into a hellfire of trouble, and a treasured imaginary friend.
You may be freaked out by such episodes, and there is no doubt that they turn 'Gainsbourg' into a bitty and whimsical affair. On the other hand, I would back anything that loosens the bonds of the bio-pic."
- Anthony Lane, "Private wars: 'The Debt' and 'Gainsbourg'", 12 September 2011 The New Yorker
No, still not quite sure what he means.