A character tenor with a vast repertoire of roles, Castel was also a gifted linguist. In 1989, he joined Juilliard's Vocal Arts faculty as a diction coach and also taught a course on the history of singing. He retired from Juilliard in 2014.

"I have been honored for 28 years to be an esteemed colleague [at Juilliard and at opera houses and festivals around the world] of Nico Castel, and more importantly his friend," Vocal Arts faculty member Corradina Caporello wrote in an email to The Journal. "We spent long hours on the phone trying to untangle some of the most mysterious meanings of archaic Italian." Castel had a remarkable ability to imitate the accents and inflections of the famous singers he coached and with whom he performed, she said.

Nico Castel was born Naftali Chaim Castel Kalinhoff in Lisbon in 1931 and raised in Venezuela. He had a German governess and attended a French school, ultimately acquiring native fluency in English, Spanish, Portuguese, French, German, and Italian along with facility (and the ability to sing and coach song repertoire) in Ladino and Yiddish. Prior to his singing career, he sold toilets for American Standard in Latin America, worked for a whiskey importer in Venezuela, and served as an interpreter of German and French for the U.S. Army. Castel immigrated to the U.S. in 1948 to pursue singing and made his recital debut at Town Hall in 1958, his prize for winning the first Joy in Singing competition. The following year he was a guest artist for a Juilliard Opera Theater production of The Count Ory, an English adaptation of Rossini's opera.

In 1970, Castel made his debut with the Metropolitan Opera. In the almost 40 years of his Met career (27 of them as a performer), he sang 57 roles in 793 performances and also was a staff diction coach, translated opera librettos prolifically, and wrote manuals on diction for singers. He was one of the company's leading comprimario, or character, tenors—singers who play supporting and often comic roles in an opera. Toward the end of his career, he focused on spoken roles like the Major-Domo in Richard Strauss's Ariadne auf Naxos and Lillas Pastia in Carmen. A YouTube video with more than 16,000 views as of this writing features Castel reciting in seven dialects the first words of the Major-Domo's opening speech.

Castel's legacy includes tomes of opera libretto translations, which consist of word-by-word English translations and phonetic symbols for each word in the original language. Affectionately known by singers and coaches as "the Nico books," these include the complete librettos of operas by Mozart, Verdi, and Puccini, as well as three volumes of French opera librettos, all published by Leyerle.

A former cantor, Castel also performed with the former New York City Opera, Lyric Opera of Chicago, and the Santa Fe, San Francisco, and Seattle operas. And he founded the New York Opera Studio, a training program for young singers, with his third wife, Carol Castel, who, with his daughter and half-brother, survives him.