25th SAG Awards is a Fashion-Forward Union Behind The Scenes

The 25th SAG Awards is in full swing, and if you take a peek behind the scenes ... you'll see everyone was dressed to the nines and beyond for this thing.

Tons of actors were on hand Sunday for the award show honoring folks in the SAG-AFTRA union, and nobody skimped on the swank -- including Michael B. Jordan (who wore a colorful Louis Vuitton harness over his tux), Lady Gaga who dazzled in a white gown, Viggo Mortensen who killed it with maroon ... and lots and lots of other stars who brought out their best outfits.

Some other standouts ... Chris Pine in the classic all-white tux, Sandra Oh who donned a gorgeous red gown, Darren Criss who sizzled in silver and, frankly, the entire "Black Panther" cast ... who looked absolutely incredible together in the front row.