(CNN) -- There's no doubt about "Doubt."
Both Meryl Streep and Philip Seymour Hoffman earned nominations for "Doubt."
The movie version of John Patrick Shanley's Tony Award-winning drama about sexual and religious power struggles at a Bronx Catholic school in 1964, led the nominations for the 15th annual Screen Actors Guild Awards with five, including nods for each major member of its cast and an overall pick for outstanding performance by a cast.
Meryl Streep, who plays a suspicious school principal, Sister Aloysius, in the film, was nominated in the lead actress category. Her co-stars, Philip Seymour Hoffman, Amy Adams and Viola Davis, earned nominations in supporting categories.
Also doing well at the SAG Award nominations were "The Curious Case of Benjamin Button" and "Milk," both of which earned three nominations. The films are up for outstanding cast, lead actor (Brad Pitt and Sean Penn, respectively) and a supporting performance.
Kate Winslet was nominated for her performances in two films. The actress earned a lead actress nomination for "Revolutionary Road" and a supporting nod for "The Reader."
The late Heath Ledger, who earned critical and mass praise for his performance as the Joker in "The Dark Knight," was nominated for supporting actor.
The outstanding cast nominations went to "Button," "Doubt," "Frost/Nixon," "Milk" and "Slumdog Millionaire."
The SAG Award nominations also shined a light on a handful of lower-profile performances. Richard Jenkins, who received plaudits for his performance in the little-seen "The Visitor," received a best actor nomination. Melissa Leo, who plays a struggling single mother who turns to smuggling in "Frozen River," earned a best actress nomination.
The SAG Awards also honor TV series, and among the top nominees are "Boston Legal," which earned nominations for stars William Shatner, James Spader and its ensemble; "Mad Men," which picked up nods for Jon Hamm, Elisabeth Moss and its ensemble; and "30 Rock," which received nominations for Tina Fey, Alec Baldwin and its ensemble.
For Academy Award watchers, the SAG Awards nominations are considered a bellwether of Oscar acting nominations. The actors' branch is the largest Academy voting bloc and is responsible for creating the short lists for the various Oscar acting categories. iReport.com: What do you think of the nominations?
However, a SAG Award win is by no means a guarantee of an Oscar. Though last year's SAG Award lead actor winner, Daniel Day-Lewis, won the best acting Oscar for his performance in "There Will Be Blood," the SAG Award lead actress winner, Julie Christie, didn't duplicate the feat. Christie, who was nominated for "Away From Her," lost to Marion Cotillard, who played Edith Piaf in "La Vie en Rose."
Moreover, SAG Award nominees for casts sometimes have little overlap with the Oscar nominees for best picture. Last year, the SAG Award list had just one film -- eventual winner "No Country for Old Men" -- that was also nominated for best picture.
SAG Award nominations are decided by about 4,000 randomly selected members of the actors' union. The full membership is allowed to vote on the winners.
The 15th annual SAG Awards are scheduled to air on January 25 on TNT and TBS. Both networks are units of Time Warner, as is CNN.