“Fun Size” thesp Thomas Middleditch has joined the cast of “Rivals,” the Jay Roach-directed Warner Bros. comedy that’s currently shooting in New Orleans.Will Ferrell and Zach Galifianakis star as rival politicians running for a congressional seat in a small North Carolina district election. Middleditch will play a slacker who works with Galifianakis’ character. The rest of the pic’s supporting cast includes Dylan McDermott, Jason Sudeikis, John Lithgow and Brian Cox. Earlier this year, Middleditch was named one of Variety’s 10 Comics to Watch, as he regularly performs at the UCB Theatre and in College Humor videos. Thesp, who will soon be seen alongside Paul Dano and Robert De Niro in Paul Weitz’s “Being Flynn,” is also attached to star in Scot Armstrong’s comedy “Road to Nardo.” Additionally, Middleditch just received a pilot order from MTV for his animated project “Worst Friends Forever” that he will voice and exec produce with Mike Judge, John Altschuler and Dave Krinsky. Middleditch is repped by WME and Kirsten Ames Management.