I don't know much about this, but I had the same question because the guy on the Letterman bit seems so weird. I Googled Singletary + David Letterman, and got an article which explains that Letterman is doing a semi-regular, fake interview with an actor portraying Singletary. It is not actually Singletary. I hope that helps.
The actor is named Art Kelly and has been in other skits as himself with David.
There has only been one actor who played Eli David and that actor is Michael Nouri.
David Warner
His character was Ross, and the actor is David Schwimmer
PCH: Russell Crowe
David Hyde Pierce
Gerard Mulligan has: Played himself in "The David Letterman Show" in 1980. Played Gerry - Gives Money-Saving Tips in "Late Show with David Letterman" in 1993. Played Himself - Former Staff Writer in "Late Show with David Letterman" in 1993. Played Somali Pirate in "Late Show with David Letterman" in 1993. Played Master Artist - Painting a Presidential Portrait in "Late Show with David Letterman" in 1993. Played Himself - Gives Green Tips in "Late Show with David Letterman" in 1993. Played Abigail Smith Adams in "Late Show with David Letterman" in 1993. Played Gerry - Green Living Expert in "Late Show with David Letterman" in 1993. Played himself in "Late Show with David Letterman" in 1993. Played Scooter Libby in "Late Show with David Letterman" in 1993. Played PC Computer in "Late Show with David Letterman" in 1993. Played Himself - Taking a Break from Picketing in "Late Show with David Letterman" in 1993. Played Snow Shovel Murderer in "Late Show with David Letterman" in 1993. Played Eliot Spitzer in "Late Show with David Letterman" in 1993. Played Karl Rove in "Late Show with David Letterman" in 1993. Played Friend of President John Adams in "Late Show with David Letterman" in 1993. Played President Bill Clinton in "Late Show with David Letterman" in 1993. Played Various Characters (1993-2004) in "Late Show with David Letterman" in 1993. Played Himself - NYPD Ride-Along with Chris Elliott in "Late Show with David Letterman" in 1993. Played Paris Hilton in "Late Show with David Letterman" in 1993. Played Himself - Movie Actor in "Late Show with David Letterman" in 1993.
Mark Delabarre has: Played Jimmy in "Ernest Scared Stupid" in 1991. Played Various Characters in "Late Show with David Letterman" in 1993. Played Hardees CEO in "Late Show with David Letterman" in 1993. Played Lawyer in "Late Show with David Letterman" in 1993. Performed in "Late Show with David Letterman" in 1993. Played Lottery Winner in "Late Show with David Letterman" in 1993. Played Bruce in "Ed" in 2000. Played Father in "Highway 403, Mile 39" in 2004. Performed in "Spatula Boy" in 2004. Played FBI Agent in "Z Rock" in 2008. Played Priest in "Royal Pains" in 2009. Played Animated Actor - surgeon in "Youth Knows No Pain" in 2009. Played Simon in "8:46" in 2011. Played Gibbs in "Believe" in 2014.
Russell Crowe
Ross Geller was played by actor David Schwimmer.
There has only been one actor who played Eli David and that actor is Michael Nouri.
The character David Gray was played by Paul Rossilli.
David Spinx David Spinx
David Warner
Russell Crowe
His character was Ross, and the actor is David Schwimmer
PCH: Russell Crowe
Chris Elliott is an American actor. He has appeared on Late Night with David Letterman, Get A Life, Everybody Loves Raymond, and How I Met Your Mother.