AUSTIN, Texas (Reuters) - Gary Clark Jr. was shaking. He was around 15, with the X's of an underage club patron marking the backs of his hands. He'd brought his guitar to the Austin blues institution Antone's and had just been called on stage to play alongside Hubert Sumlin, Calvin "Fuzz" Jones, James Cotton and Mojo Buford.
"They were some of the most low-down guys in blues," Clark, now 28, told Reuters during the South by Southwest (SXSW) music festival in the Texas capital, where he was born and still lives. "It was kind of a special moment, just kind of the beginning of all this."
For Clark, all this includes, just in the past few months, playing on the "Late Show with David Letterman", opening for Eric Clapton in Brazil and performing for President Barack Obama and Michelle Obama during a White House blues showcase that also featured B.B. King, Buddy Guy and Mick Jagger.
All from a guy who failed a public speaking assignment in high school because he was too terrified to show up for his presentation. Even now, the tall, lanky blues-soul-rock musician said, "everyone's always telling me to speak up."
Still, those who have been watching him for years in Austin say he's evolved from a shy performer to a confident band leader. He pivots from mellow songs - sometimes singing falsetto - to loud, hot displays of guitar intensity, as he did during a late-night show last week during SXSW.
"You gonna know my name by the end of the night," Clark, wearing a black hat, sang as part of the title song of "Bright Lights", his major-label debut EP from Warner Bros.
JAMMING IN THE GARAGE
Clark started playing guitar at age 12, a year after his friend Eve Monsees, whose house was so close to his that he could hear her jamming down the street. They started playing in their garages after school.
"Sometimes, we'd open the door, and there'd be people outside clapping and stuff, just seeing these kids play," Monsees said.
To celebrate her 15th birthday, the pair played at a blues jam at Babe's on Sixth. At that point, their only performance had been at their middle-school talent show.
"I didn't know about blues jams or anything," Clark said. "I was just kind of hooked from there. I loved being on stage, I loved the music."
They returned regularly. Together, Clark and Monsees would explore places like Antone's Record Shop (of which she is now a co-owner), discovering Magic Sam, Otis Rush, Freddie King and Hound Dog Taylor. Their dads would take them to hear live music. Local musicians including Jimmie Vaughan - brother of the late Stevie Ray Vaughan - took Clark under their wing.
Clark was a teenager in 2000 when Austin music writer Michael Corcoran first saw him play.
"He was a big flash of energy and excitement," said Corcoran. "A lot of times, blues can be pretty boring. It was just electrifying."
But Clark also understood restraint, Corcoran said. Unlike many people his age, he resisted playing everything he knew in every solo.
'INSANE' GUITAR SKILLS
Clark fan Joel Boehm, an Austin lawyer, saw him play dozens of Wednesday nights at Austin's Continental Club several years ago. During the guitar solos, the audience would stand entranced, not talking, just fixated on the music, Boehm said. He said he's watched Clark become more confident, engaging more with the audience.
"It's not like he has the rock-star swagger," Boehm said after watching one of Clark's SXSW shows. "It's that look in his eyes, like ‘I know something you don't.'"
Some high-profile people are noticing, notably since he played Clapton's Crossroads Guitar Festival in 2010. Alicia Keys has gushed about him.
"Even if you're not listening to blues, this guy's on everyone's radar right now," said Wendy Geller, a Los Angeles-based music blogger for Yahoo! Music. "He's got pretty insane guitar skills."
Clark has also dabbled in acting, appearing in the 2007 movie "Honeydripper" with Danny Glover. But he said he realized he'd have a lot of work to do on acting in order to seriously pursue more roles.
Meanwhile, Clark - who still pops into the blues jam on Sixth Street (the place is now called Friends) - is at work recording a full-length album. And this year, he's set to play at Coachella in California, at New Orleans' Jazz Fest and at Bonnaroo in Tennessee.
"I kind of feed off of the energy of the people," he said of live audiences. "It's like, oh yeah, we're all in this together, we're all just kind of going through this crazy life together, and for a few minutes, we can come together, and just let it go and have a good time."
(Editing by Jill Serjeant)