Final answer:
Leonard Bernstein began his career as a substitute conductor for the New York Philharmonic on short notice. Meanwhile, other composers like Arnold Schoenberg were pushing the boundaries with atonality, while American musical theatre flourished with works from Gershwin, Rodgers, Hammerstein, and Cole Porter.
Step-by-step explanation:
The composer, conductor, and pianist who began his spectacular career as substitute conductor of the New York Philharmonic on only a few hours' notice was Leonard Bernstein. Bernstein was a dynamic and multifaceted musician who not only made significant contributions as a conductor but also as a composer, influencing the world of classical and musical theatre. He is notably known for his works such as ‘West Side Story’ and his involvement with the New York Philharmonic.
Contrastingly, other composers like Arnold Schoenberg were making bold moves by challenging the traditional scales and audiences' expectations with atonality and the twelve-tone scale. This was a deliberate subversion of the tonal music tradition since the Renaissance. Meanwhile, during the 1930s and 40s, American musical theatre witnessed a resurgence with legendary composers and lyricists such as George and Ira Gershwin, Richard Rodgers with Oscar Hammerstein, and Cole Porter, offering a much-needed distraction with their iconic musicals during challenging times for America.