How do we find the next Pinter or Stoppard? Should phones be banned at the theatre? Does the National need a base in the north? As NT boss Indhu Rubasingham announces her enticing second season, with a major West Side story revival, a new play about John Major and star turns from Emma Corrin and Emilia Clarke, she speaks exclusively to Jessie Thompson about the challenges – and thrills – of the biggest job in British theatre