Messalla was part of a TV crew that filmed propos for the war as part of the Airtime Assault. He was the assistant to the director, Cressida. He was originally from the Capitol but had joined the rebels in the war.
When Katniss' sharpshooter squad, 451, was selected for a mission to go to the Capitol, the TV crew came along as well to continue to film propos for the war. At one point during the mission, the team was forced to find refuge in an abandoned apartment which bore the same floorplan as Messalla's previous residence in the Capitol. Messalla proved himself most valuable because he knew where people would most likely stash food in the hidden storage spaces. As a result, the team found canned goods. He was also Katniss' make-up helper for the propos.
When Boggs, the commander of Squad 451, triggers the bomb that blows off his legs, the explosion throws Messalla into a wall and knocks him out. Fortunately, he is revived by Finnick Odair and a few others before continuing with the mission.
Later in the mission when the team was underground in the Transfer and running from the lizard mutts, Messalla fell prey to an unnamed pod. He was trapped in a wide shaft of golden light that radiated from the ceiling to the floor. Gale's arrows were unable to penetrate it and Messalla was as still as a statue, poised up on the ball of one foot, head tilted back, mouth stretched wide as if he was yelling. Inside the shaft of light, his flesh melted off his body like candle wax. The other members were unable to do anything but watch him slowly die. Cressida was upset by his death, as he was her assistant, and friend.
In The Hunger Games: Mockingjay - Part 2, his death is similar to in the book as he is hit by a beam of light coming down from the ceiling whilst the team are running through the underground car park. It's possible by the design of the car park that the beam of light was in fact a pillar which could appear and dissapear which re-appeared when Messal;a was running there, killing him.
Messalla was described as being a slim young man with several sets of earrings. His tongue had also been pierced, and he wore a stud with a silver ball the size of a marble. In the film, his appearance is much the same, except he also has several piercings in his lower lip and one in the bridge of his nose.