Springtime casting has begun to firm up at a number of Broadway productions, including the addition of Justin Kirk to the cast of play “Other Desert Cities” and of Michael Urie to tuner revival “How to Succeed in Business Without Really Trying.”

Also on tap: Brit actor Tam Mutu and Karen Mason (“Wonderland”), among the thesps to join Sierra Boggess in new tuner “Rebecca,” and most of the Boston company of the new revival of “Porgy and Bess,” now locked in to move with that production to the Main Stem.

Kirk (“Weeds”) takes over for Thomas Sadoski in “Desert Cities,” the box office success of which recently prompted producers to extend what was originally a limited engagement into an open-ended run. Sadoski — who also appeared in the show’s Off Broadway preem at Lincoln Center earlier this year — will exit the Broadway transfer in January in order to work on the new HBO skein from Aaron Sorkin, “The Newsroom.” (His tube commitments will also require him to be away from the show for a few weeks this month, when understudy Matthew Risch will play the part.)

Kirk, who’ll portray a reality-TV producer at an explosive family gathering, steps into “Desert Cities” Jan. 10.

Urie (“Ugly Betty”) is set to join “How to Succeed” in late January, appearing alongside fellow replacements Nick Jonas and Beau Bridges. Urie will replace Christopher J. Hanke as Bud Frump, the boss’s nephew who schemes to undermine the rise of protag J. Pierrepont Finch.

Urie starts perfs in the tuner Jan. 24, as does Jonas.

In “Rebecca,” the new tuner set to open in April, Mutu (“Chess” in Toronto, “Love Never Dies”) will play the lead character’s mysterious new husband, while Mason is onboard as the unnerving housekeeper Mrs. Danvers. James Barbour (“A Tale of Two Cities”), John Dossett, Donna English and Nick Wyman also have booked gigs in the show, which begins previews March 27 ahead of an April 22 opening at the Broadhurst.

As for “Porgy,” it had seemed likely that much of the cast that appeared in the rejiggered revival’s Boston tryout earlier this year would come to Gotham with topliners Audra McDonald, Norm Lewis and David Alan Grier. Now confirmed are Phillip Boykin, Joshua Henry (“The Scottsboro Boys”), Nikki Renee Daniels, Christopher Innvar, Bryonha Marie Parham and NaTasha Yvette Williams, among others. Show begins previews Dec. 17 for a Jan. 12 opening at the Richard Rodgers.