Famous Instrumentalists Who Thought They'd Try Conducting...

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Arturo Toscanini...
participated as cellist in the world premiere of Verdi's Otello (La Scala, Milan, 1887) under the composer's supervision. Verdi, who habitually complained that conductors never seemed interested in directing his scores the way he had written them, was impressed by reports from Arrigo Boito about Toscanini's ability to interpret his scores. The composer was also impressed when Toscanini consulted him personally about the Te Deum, suggesting an allargando where it was not set out in the score. Verdi said that he had left it out for fear that "certain interpreters would have exaggerated the marking" (wikipedia)