Mackey was announced Jan. 6. The previously announced cast includes Tony Award winner Barbara Cook, Vanessa Williams, Tom Wopat, Leslie Kritzer, Norm Lewis, Euan Morton and Matthew Scott.

James Lapine, Sondheim's collaborator on Into the Woods, Passion and Sunday in the Park With George, conceived Sondheim on Sondheim and directs it. The score draws from the catalog of the Tony Award-winning Sondheim, whose Broadway scores include A Little Night Music, Follies, A Funny Thing Happened on the Way to the Forum, Sweeney Todd, Merrily We Roll Along, Pacific Overtures, Company and more.

Opening night is April 22. This will be a limited engagement through June 13.

Tickets for Sondheim on Sondheim are currently on sale through an American Express pre-sale offer and go on sale to the general public on Jan. 10.

According to Roundabout notes, "By writing songs that reflect the complexity of his characters, he has changed the way we define a great musical. But even though millions of fans know his songs by heart, few know much about Stephen Sondheim himself. Until now. Roundabout presents Sondheim on Sondheim, an intimate portrait of the famed composer in his own words... and music. Through the use of exclusive interview footage, you'll get an inside look at Sondheim's personal life and artistic process. An ensemble cast of Broadway's best will perform brand-new arrangements of over two dozen Sondheim tunes, ranging from the beloved to the obscure. Directed by frequent Sondheim collaborator James Lapine, this unique experience will take you inside the life and mind of an ordinary New Yorker… with an extraordinary talent." The Sondheim on Sondheim creative team includes David Loud (music direction and arrangements), Dan Knechtges (musical staging), Beowulf Boritt (sets), Susan Hilferty (costumes), Ken Billington (lights), Dan Moses Schreier (sound) and Peter Flaherty (projections).

Roundabout has produced five Tony Award-nominated Sondheim revivals including Company (1995), Follies (2001), the Tony Award-winning Assassins (2004), Pacific Overtures (2004) and Sunday in the Park with George (2008).

Tickets are currently available exclusively to American Express Cardmembers at www.roundabouttheatre.org, by phone at (212) 719-1300, or at the Studio 54 Box Office (254 West 54th Street). General public on-sale begins Jan. 10 by calling Roundabout Ticket Services at (212) 719-1300, online at www.roundabouttheatre.org or at the Studio 54 Box Office (254 West 54th Street).

Sondheim on Sondheim will play Tuesday through Saturday evenings at 8 PM with a Wednesday, Saturday and Sunday matinee at 2 PM.

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Barbara Cook last appeared in a Broadway musical in 1971 in The Grass Harp. She won the 1958 Tony Award as Best Featured Actress in a Musical for playing Marian in The Music Man. She is known for her interpretations of the work of Sondheim and received a Tony Award nomination for Best Theatrical Event for her solo, limited engagement concert, Mostly Sondheim, in 2002. A new collection of her recordings, "The Essential Barbara Cook," was recently released on DRG. For the past 30 years, she has been acclaimed for her concert and cabaret work.

Vanessa Williams received a 2002 Tony Award nomination for her performance of the Witch in Into the Woods and made her Broadway debut in 1994 in Kiss of the Spider Woman. She's a regular on the TV series "Ugly Betty."

Tom Wopat's Broadway credits include A Catered Affair (Tony, Drama Desk & Drama League nominations), Chicago, Glengarry Glen Ross (Drama Desk Award), 42nd Street, Annie Get Your Gun (Tony & Drama Desk nominations), Guys and Dolls, City of Angels, I Love My Wife, and Off-Broadway's The Guys, Olympus on My Mind, A Bistro Car on the CNR, Oklahoma.

Leslie Kritzer's Broadway credits include A Catered Affair (Drama Desk nomination), Legally Blonde (Clarence Derwent Award for Most Promising Performer) and Hairspray. She received critical acclaim and a Time Out New York Award for her sold-out Joe's Pub run of Leslie Kritzer Is Patti LuPone At Les Mouches (a performance she repeated at The Plush Room in San Francisco). Off-Broadway and regional credits include Rooms, On The Town (Encores!), The Great American Trailer Park Musical (Drama Desk nomination), Bat Boy, Godspell, Broadway: Three Generations (Kennedy Center), Cabaret, Born Yesterday, the world premiere of Vanities, Urinetown (national tour), Evita and an acclaimed Fanny Brice in Funny Girl.

Norm Lewis' Broadway credits include The Little Mermaid (King Triton); Les Misérables (Javert - Drama League nomination); Chicago (Billy Flynn); Amour; The Wild Party; Side Show (Jake); Miss Saigon (John); and Tommy. Off-Broadway credits include Dessa Rose (Nathan - Drama Desk nomination, AUDELCO Award); Two Gentlemen of Verona (Valentine - Drama League nomination); Captains Courageous (Doc); A New Brain (Roger).

Euan Morton received an Olivier Award nomination for originating the role of Boy George in the musical Taboo. He reprised the role on Broadway, earning Tony, Drama Desk and Outer Critics Circle nominations, as well as the Theatre World Award (for Outstanding Broadway Debut). Other stage appearances include title roles in Tony Kushner's adaptation of Brundibár at the New Victory Theatre and Berkeley Rep and The Who’s Tommy at the Bay Street Theatre. He earned a 2006 Obie Award for his performance in Measure for Pleasure at the Public Theater. He performed opposite Alfred Molina in Howard Katz for the Roundabout Theatre Company. He was most recently seen in Moisés Kaufman's production of Into the Woods at the Kansas City Rep.

Matthew Scott's Broadway credits include Jersey Boys, A Catered Affair and The Best Little Whorehouse in Texas (for The Actors Fund). His regional credits includes The Light in the Piazza (Fabrizio, Philadelphia Theatre Co.), First You Dream: The Music of Kander and Ebb (Signature Theatre), Ace (Signature Theater), Ragtime (Houdini, Paper Mill Playhouse) and more.

Mackey recently made her Broadway debut as Glinda in Wicked after having performed the role in both the Chicago and Los Angeles companies. Growing up in Orange County, she performed at Saddleback CLO and Fullerton CLO and won the Los Angeles Music Center Spotlight Awards competition at the Dorothy Chandler Pavilion. Film/TV credits include "The Parent Trap" (directed by Nancy Meyers), "Do Over" and "Family Affair." She took part in the most recent workshop of Sondheim on Sondheim.