John C. Reilly ain't just your average movie star -- he's got some decent vocal chops to boot ... with this impromptu performance giving us major flashbacks to one of his roles.
The actor cowboyed up on stage at L.A.'s Fonda Theatre Thursday night ... surprising everyone during Sierra Ferrell's country set -- and as you can see, the crowd goes wild for his performance ... which seems to have come out of nowhere.
John was clearly having fun with this ... he's going full-on wacky and animated for this -- and the crowd can't get enough of him, it seems.
As for John and Sierra -- it's unclear what their connection is and how they actually know each other, but he joined her the previous night onstage as well.
Of course, fans of John's probably felt like he was channeling Dewey Cox from 'Walk Hard' here -- and he may well have been doing that. Either way, we know JCR is a showman.
This ain't the first time John's stolen the spotlight with his vocals -- back in October, he linked up with his "Step Brothers" costar Will Ferrell to belt out the movie's iconic track "Boats & Hoes" ... with a little help from none other than Snoop Dogg.
Take it away, John!