The loosely scripted, improv-style comedy “Party Down” (9:30 p.m., Starz) revolves around six cater-waiters and employees of the Party Down Corp., who work the local Hollywood circuit while awaiting their next big break.

Adam Scott stars as Henry, an actor who made one notable commercial with an annoying catch phrase that haunts him wherever he goes. A real mope in a rumpled shirt, he’s particularly miserable after returning to the bartending scene after eight years of quasi-celebrity. Now sober, Ron (Ken Marino) dreams only of being the best cater-waiter he can be, and that makes him the most delusional character of all.

Jane Lynch (“40 Year Old Virgin”) plays Constance, an unabashed eccentric whose three-decade-long career as an extra got off to a bad start when she was crushed by a chandelier during the shooting of “The Poseidon Adventure.” Always capable of infusing “normal” characters with a manic energy and a propensity for blurting out the unexpected, Lynch is simply one of the funniest actors working.

In the debut episode, Enrico Colantoni (“Flashpoint”) cameos as affluent estate lawyer who can’t stand his career, his life, his pleated shorts or the fact that his wife has thrown the party for the Willow Canyon Homeowners Association and plans to give out awards for best lawn, best Christmas lights, best mailbox, etc.

The show’s loose style, absurd banter and the forced “are we having fun yet?!” atmosphere of professional party people only serve to heighten its overarching sense of personal and professional disappointment. As fans of “The Office” know, it takes a certain intelligence and confidence to mingle the funny and the tragic so easily and awkwardly.

l In contrast, the unscripted “Head Case” (9 p.m., Starz, second-season premiere) tries a little too hard when it really doesn’t have to try at all. Alexandra Wentworth stars as celebrity therapist Dr. Elizabeth Goode. A thin plot about her engagement and office travails provides just enough background for sessions with real-life celebrities from Hugh Hefner to Jerry Seinfeld. But in each session, Goode seems to talk more about herself and her needs than her patients. Tonight’s episode features singer Macy Gray and actresses Janeane Garofalo and Tori Spelling. The celebrity guests seem to have fun during their sessions, but the sexual banter on “Case” often goes the way of much premium cable fare, undercutting some smart performances with the needlessly crude.

l Milestones for the night include the fourth season and series finale of “Battlestar Galactica” (8 p.m., Sci Fi), a two-hour episode.

Tonight also marks the 500th installment of “Friday Night SmackDown!” (7 p.m., MyNetwork) that I haven’t watched.

Tonight’s other highlights

l Jesse’s submarine mission in the future takes a dangerous turn with fateful implications for John, Sarah and Derek on “Terminator: The Sarah Connor Chronicles” (7 p.m., Fox).

l Joe wants J.D. to keep his head in the game on “Friday Night Lights” (8 p.m., NBC).

l While playing the perfect wife for an Internet tycoon (Patton Oswalt), Echo encounters Agent Ballard on “Dollhouse” (8 p.m., Fox).

l Trudi clings to Richard on “Mistresses” (8 p.m., BBC America).

l Eileen Davidson (“Young and the Restless”) has questions for “The Dog Whisperer” (8 p.m., National Geographic).