A funny thing happened to Hildy Parks the other night on her way to the opera. Hildy, the writer-actress wife of Broadway impresario Alexander H. Cohen, was detoured by her husband to the elegant St. Regis Roof in Manhattan for a cocktail. When she strolled in, 186 people leaped up, shouted SURPRISE! and broke into a ragged chorus of "Happy Birthday."

For Hildy, a former regular on the TV sudser Love of Life, it was a lovely way to turn 50. For three months her husband, Alex (Beyond the Fringe, Ulysses in Nighttown, 6 RMS RIV VU), had been covertly planning the glittering soiree. Old friends were recruited and sworn to silence, among them Richard Burton, Celeste Holm, Mike Nichols, Peter Ustinov, Walter Slezak, Christopher Plummer, Alfred Drake, Anne Jackson, Eli Wallach and John V. Lindsay.

Cohen's plot was all the more impressive because during the workday his wife inhabits an adjoining office, where she is writing the script for his upcoming Tony Awards epic on ABC-TV. The Cohens have teamed up on the show for the past 10 years, half of their married life. Their other collaborations include two sons, the elder of whom, 19-year-old Gerry, was flown in secretly by his father from London.

Burton and his current girlfriend, London model Suzy Hunt, drew a good deal of attention, even in supporting roles. Burton was the only man in the room without a black tie—he wore an ancient Nehru jacket. Suzy sported a sweeping Oscar de la Renta number. "Where did you get your dress?" someone asked. "It's American," Suzy answered. "And damned expensive it was," added Burton, looking up from his rack of lamb and Perrier water. "I paid for it."

Despite the stars, it was Cohen's show, and he topped it off with a champagne toast to his wife. "She is witty, restless, energetic, efficient, organized, impatient, logical, concerned, informed, curious, thrifty, loyal, funny, caring, loving and a great hostess. And now that she's 50, I can finally admit she's a great cook, too."

Hildy had the last word. "A friend on a soap opera 22 years ago said to me when she met Alex: 'Honey, you've found Santa Claus. Grab him.' "