From an earlier answer

Talleyrand was very much a part of the liberal wing of the French Revolution; aristocratic Enlightenment thinkers that wanted reforms such as a constitution and a hierarchical based society, but not fundamental alterations of France itself. This was partly why Talleyrand himself survived the upheavals of the Revolution. He was enough of a friend to the Revolution that his noble and church background did not make him an easy target, but his moderation meant that he became a figure the Directory courted after Thermidor. French liberals at first saw Napoleon as a solution to the Revolution in that a strong Enlightenment figure could restructure France long rational lines. These liberal thinkers increasingly soured on Napoleon as the Consulate transitioned into Empire and Napoleon became more dictatorial.

Although Talleyrand was an important early collaborator and part of the Napoleonic state apparatus, his relationship with Napoleon was increasingly strained after the latter declared himself emperor. Talleyrand felt that Napoleon's territorial aggrandizement was too excessive and sabotaged Talleyrand's attempts to forge a new balance of power on the continent. After Talleyrand's voluntary resignation as Foreign Minister in 1807, his salon and social circles became one of the main areas in which opposition to Napoleonic rule inside France percolated. Talleyrand's wealth and social cachet created a safe space for them to vent their frustrations with the Empire. Talleyrand knew to judiciously limit his direct participation in seditious activities, which imparted to him knowledge of political opinion amongst elite French circles while never giving Napoleon a pretext to imprison him. It's important to note that Napoleonic rule, although dictatorial, was seldom vindictive or arbitrary as a whole (the drumhead execution of Duke d'Enghien being a notable exception) and the Napoleonic state allowed this soft dissent so long as it remained behind closed doors and not evolve into public rebellion.

These social connections became vital for Talleyrand when it was becoming increasingly apparent that Napoleon's fortunes had abandoned him in 1813. As the domestic foundations of Napoleonic rule foundered, Talleyrand began to play an increasing role as a public individual in these times. Napoleon had never fully stripped away his titles and this became the venue by which Talleyrand became a major partisan for the Bourbons. The abortive Malet conspiracy of 1812, which forged a Senate decree declaring Napoleon was dead in Russia, gave Talleyrand the idea that the Napoleonic Senate had the legal authority to depose the Emperor. Although the Senate had often been a servile rubber stamp for Napoleon's autocracy, it had the legal potential to declare a new state. Talleyrand, still a Senator, convened the legislative body after he had sounded out like-minded conspirators in his salon, and declared a provisional government.

Although the Bourbons did not trust Talleyrand, they discovered he was one of the few individuals who possessed both broad diplomatic experience and vast social capital within France. In short, the Bourbons saw greater utility in employing Talleyrand than score settling. Talleyrand's famous charisma became very important convincing the other Allies not to back a regency under Marie-Louise, which would have potentially damaged French sovereignty and instead restore the Bourbons.

Talleyrand's motives for the Restoration are more murky. There certainly was a degree of realism that underlay his actions. Although the Napoleonic legend has added luster to Napoleon's handling of the military campaign in 1814, France's strategic situation in 1814 was quite dire. Napoleon's diplomatic overtures to the Allies were seldom realistic and premised on negotiating from a position of military strength that was fast evaporating. Talleyrand's political ambition also played a role. Talleyrand had been out in the political wilderness for some time and the chaos of 1814 presented him an opportunity to rectify this situation. There is some evidence that he initially wanted a Bonapartist regency in which he would become the Prime Minister and control the young Napoleon II, but Marie-Louise's flight from the capital made the Bourbons his next logical alternative. Like many politicians, Talleyrand kept his options open and was able to adapt his schemes to suit the needs of the moment.

These instincts served Talleyrand quite well and earned him the reputation of being a survivor. This meant that the post-1815 political factions in France saw more symbolic value in courting his favor than targeting him as one of the elder statesmen of France.

Sources

Dwyer, Philip G. Citizen Emperor: Napoleon in Power. New Haven: Yale University Press, 2013.

Price, Munro. Napoleon: The End of Glory. New York: Oxford University Press, 2014.

Woloch, Isser. Napoleon and His Collaborators: The Making of a Dictatorship. New York: W.W. Norton, 2001.