We talked to Stephen Glickman -- who's already famous from the "Big Time Rush" show -- on "The TMZ Podcast," where he discussed his now-viral TikTok sound ... in which he does a slow, dramatic rendition of Gnarls Barkley's 2006 hit, "Crazy." Tons of stars have hopped on the trend -- lip-syncing to his track and sharing something cringe-worthy from their past.

There's nothing cringey about Stephen's success, however ... 'cause he says he's set to perform a WNBA halftime show soon. SG tells us he and his band are gonna fly out to Las Vegas ... where he will perform for the sold-out crowd at the Aces vs. Minnesota Lynx game.

Stephen says he's also booked something with HGTV as a result of the newfound fame -- so yeah, it sounds like things are rolling ... and it's all thanks to this silly video he posted.

BTW, for those who may not know ... Stephen also is a comedian and played a record producer on the 'BTR' show, too ... making his new music venture somewhat a full circle moment.

While his stardom appears to be on the rise, Stephen admitted he's yet to see any dough from the hot track just yet ... but already has a plan for how to spend the money when it comes in. He's not really owed any royalties since he didn't write the OG, but presumably, his video has been monetized and TikTok ... and there's some money to collect on that front.

Another cool thing we learned here ... he's been chatting with CeeLo Green -- who is one-half of Gnarls Barkley -- and Stephen says he actually plans to visit the soul singer in Atlanta.