It was like the good old Late Show days for a little while Tuesday night at L.A.’s Montalban Theatre. David Letterman took over the venue to host Netflix Is A Joke Presents: This Better Be Funny with David Letterman with special guest John Mulaney . But before welcoming Mulaney into the lights, the veteran host had another trick up his sleeve bringing out beloved Late Show band leader and good pal Paul Shaffer . The capacity crowd greeted Shaffer with a standing ovation when he turned up at the top of the event. Letterman, who recently signed on to executive produce Say Hello to Our Good Friend Paul Shaffer , an upcoming feature documentary about his longtime sidekick, traded quips with Shaffer like they did for years on late night television. “I got to go up to Moonshadows and meet Mel Gibson,” Shaffer joked as he made an exit that turned out to be relatively brief.…