How did Berlioz use orchestral colour to depict events in Symphonie fantastique?

Hector Berlioz utilized orchestral colour as a fundamental tool to convey the narrative of his Symphonie fantastique, a seminal work of program music that depicts a series of opium-induced dreams.12

Rather than relying on traditional structures alone, Berlioz employed an exceptionally large orchestra to suggest specific events and emotional states. Key aspects of his approach included:12

• Expanded Instrumentation: He incorporated then-innovative instruments, such as the ophicleide and valve trumpet, alongside expanded harp and timpani sections to achieve unique sonorities.2
• Thematic Transformation: He used the idée fixe—a recurring melody representing the beloved—which changes in character and colour throughout the movements to reflect the lover's shifting psychological state.2
• Descriptive Soundscapes: The orchestration mirrors the program's dramatic arc, from the "delights and despairs" of the first movement to the macabre atmosphere of the final "witches' sabbath," where he incorporates the Dies irae to evoke a haunting, supernatural tone.2

By treating the orchestra as a palette for storytelling, Berlioz established himself as one of history's most individual orchestrators.1