The actresses join the acclaimed firefighter series in its third season on FX, which premieres May 30.
Sarandon plays a wealthy, older woman interested in Franco (Daniel Sunjata), while Tomei plays the former Mrs. Johnny Gavin (Dean Winters).
"Our cast is great, but we're talking about two icons here and it
really elevates the work on the show just by their presence," Leary, the show's co-creator, co-producer, head writer and star, told reporters in a recent teleconference.
"I look at them on the set and go, 'Are they really on my show?' We're writing these scenes, thinking, 'Susan Sarandon is going to be saying this stuff!'" said Leary. "I'm not demeaning anybody, but when I write a speech for Lenny, I pretty much know what he's going to say and how he's going to say it."
Lenny Clarke, Leary's longtime friend, plays his Uncle Teddy on the show.