Tenspeed and Brown Shoe (1980)

Ben Vereen is E.L. "Tenspeed" Turner, an inveterate con-man and master of disguise. Jeff Goldblum is Lionel Whitney, a "brown shoe" (accountant). Through an unlikely series of circumstances, they form a detective agency. Tenspeed is the realist, while Lionel fantasizes of having adventures like the fictional private eye he idolizes, Mark Savage.