OBVIOUSLY, Belmondo’s roles differ according to the various film directors he worked with, but these differences added to the development of the actor’s real personality.
Can actors learn something about the human situation from the roles they play? Of course! And in this two-way street, a metamorphosis can also occur, where the role entered into enriches the personality of the actor.

The star-making personality is therefore an ongoing self-creation by extending out of itself and coming back, not as simply a gullible commercial dilettante of various roles, but a student and graduate of roles whose masks are now taken off.

Belmondo’s classic tradition
Indeed, Belmondo, in 1967, with his role in ‘THE THIEF OF PARIS’, under Louis Malle’s perceptive direction, entered a very metaphorical representation of the role as a ‘mask’ used to achieve some gain as an end.
Belmondo, donning a moustache in ‘The Thief of Paris’, acts as a slick professional thief in 19th Century Paris whose introduction to crime is motivated by his rejection as a spouse for the wealthy Parisian girl he loves, and whose aristocratic family marries her off instead to a man of the same wealthy class.
Belmondo develops a chip-on-the shoulder class consciousness, and becomes a suave professional house thief who starts his clandestine profession by pretentiousness and burglarizing the very mansion of his ex-girlfriend’s family, then moving onto the sumptuous chateaus and salons of Paris, where he fills his duffel bags not only with jewellery, but fancy objects and expensive cutlery.
The importance of ‘The Thief of Paris’ to Belmondo’s roles is related to two interesting factors in cinematic history.
(1) The continuity of Belmondo’s acting personality to the great American classic actor, John Garfield, and
(2) the continuity of French films from mid-20th Century onward to many of the outstanding American films of the 1940s with social roots in class conflict, and the possible corrupting influence of materialistic values.
                                          
Belmondo & Classic Hollywood
Belmondo is the living proof of creative excellence achieved in the footsteps of great actors who have laid down an example relevant to one’s own. Belmondo’s personality roles at their most gregarious and socially poignant added something to Clark Gable’s and John Garfield’s roles, but in keeping with a less heroic temperament which later 1960s modernist literature and film introduced with more thoughtful and intellectual yet quite flamboyant and socially-sensitive characterizations by actors like Marcello Mastroianni, Alain Delon, Jean-Claude Brialy, Jean-Louis Trintignant, Montgomery Clift, Steve McQueen and Laurence Harvey, among others.
Belmondo’s roles, especially since ‘The Thief of Paris’ in 1967, picked up on both the existentialist self-consciousness and social motivations which made John Garfield’s films outstanding works of cinematic art. For example, Garfield’s early antics and bubbly personality in films like ‘DAUGHTERS COURAGEOUS’ (1939), ‘TORTILLA FLAT’ (1942), or ‘DESTINATION TOKYO’ (1943) are equally mirrored in Belmondo’s own way in ‘BREATHLESS’ (1959), ‘THAT MAN FROM RIO’ and ‘BANANA PEEL’ (1964).
But by ‘The Thief of Paris’ in 1967, ‘STAVISKY’ in 1974, and ‘L’INCORRIGIBLE’ of 1975, we can now also find comparisons with later Garfield classic masterpiece films like ‘HUMORESQUE’ 1946, ‘BODY AND SOUL’ 1947, and ‘FORCE OF EVIL’ 1948, films which distinguished themselves from typical Hollywood emphasis on mere presentations of character types and stereotypical actions, without exposing or delving into the motivations behind human behaviour.
Such typical Hollywood productions are often more heroic, but of less self-questioning and intelligent inclination than Garfield’s cinematic modernism.
                                        
French films & Classic Hollywood
The probing critical intellectual viewpoint which the American cinema, in its quality decades — the 1930s, 40s, and 50s — especially, championed, and which also saw its wings clipped under paranoiac State scrutiny of the 1950s, resurfaced brilliantly in French cinema from the 1960s onward.
The loss to American culture of films which delved into the motivations and social formation of American characters  — a loss which no doubt resulted in a surfeit of sensational, violent, and antagonistic expressions in escapist films as entertainment — became a gain to European cinema, particularly French, Italian, and Polish which picked up and developed the fertile social exploration of prior exciting classic American film directors, such as John Ford, Delmer Daves, Jean Negulesco, Michael Curtiz, Nick Ray, Otto Preminger, Fritz Lang, John Farrow, Robert Aldrich, Budd Boetticher, Curtis Bernhardt, Robert Rossen, John Huston, Billy Wilder, etc.
                                      
Belmondo in ‘Stavisky’
Belmondo distinguished himself in mostly French films of the 60s and 70s, which are not imitations or remakes of such American film directors’ works, but new extensions of their inquiry into the human condition.
A good example is 1974’s ‘Stavisky’, directed by the renowned French director, Alain Resnais, in which Belmondo plays Serge Alexander (alias ‘Stavisky’), who uses charisma, bluff, and numerous phony vouchers to climb to the top of the French financial world, causing an economic crisis and scandal.
Who is Stavisky really? A false ‘actor’ in love with himself, who enjoys looking a certain way, always well-groomed, well-spoken, well-postured, etc, but nevertheless a conman who subverts the theatricality of mimetic creative acting by contaminating it as a  real-life commercial means to an end.
This is one of Belmondo’s greatest roles, agreed by many, where he exposes the contingency of his profession in a brilliant display of paradigmatic modernist self-criticism. The star-making personality of the actor has now achieved the power to expose the possibility of the corrupt subversion of the very art form he participates in.