In “About Schmidt,” Jack Nicholson steers away from his typical offbeat, teeth-gnashing and somewhat intimidating character portrayals. Instead, he inhabits the role of 66-year-old Warren Schmidt, a recent retiree and modern-day Willy Loman.

Directed by Alexander Payne (“Election”), the film begins on the day of Schmidt’s retirement from the insurance business and chronicles his slump into clumsy retirement. As he begins to adjust to the slow pace of the golden years — grimacing over crossword puzzles and buying a Winnebago — Schmidt faces sudden tragedy. His wife dies unexpectedly and his daughter announces her plans to marry the wrong man, a mullet-headed waterbed salesman named Randall.

Shattered with the sudden news, a grieving Schmidt sets out on a road trip in his new RV to Denver to attend his daughter’s wedding and hopefully convince her to call it off. As Schmidt drives cross-country, retracing his childhood roots, the film vacillates between melodramatic anguish and caustic humor.

All the while, Nicholson’s grim facial expressions restrain the great dam of anger, frustration and fear that has been welling up in Schmidt all his life. Dialogue is sparse and shallow in this quiet film, and Schmidt’s surroundings are drab and lacking. Indeed, much of the film is static, with shots of Schmidt pained face, trying to hold back his thoughts.

Nicholson’s careful performance and the single, melancholy piano score hint that there is something hiding in this film. The only insights we learn of him come from a series of letters he writes to “little Ndugu,” a Tanzanian orphan he sponsors through Childreach for only $22 a month. In the correspondence to the 6-year-old boy, Schmidt unlocks the thoughts, fears and frustrations he has harbored all his life. He is often nostalgic about the choices he made, wistfully claiming he “was going to be one of those guys you read about.”

Schmidt is a sad and quiet man, and we have never seen Nicholson this forlorn and pathetic. Not since “Five Easy Pieces” has Nicholson been this conflicted. But while that film portrays him unleashing his anger on unsuspecting waitresses over chicken sandwiches, here, Schmidt keeps it all within.

Nicholson carries the film admirably, and is only weakened by an underdeveloped and poorly acted cast of supporting characters. Next to Schmidt, they come across as half-baked caricatures of mid-westerners. Only Kathy Bates, who plays the kooky, New Age mother of Schmidt’s newly acquired son-in-law, holds her own and bares all next to Nicholson — especially in one eyebrow-raising hot tub scene.

Aside from Nicholson’s performance, the brilliance of “About Schmidt” lies in its comedic details. For example, when entering his future son-in-law’s bedroom, Schmidt spies a wall covered with “certificate(s) of completion” and colorful ribbons for “participation.” Cut to Schmidt’s facial expression: “My daughter’s marrying a loser.”

Though billed as a comedy, this is a remarkably human film with a heavy helping of internal conflict and soul-searching. Naturally, one expects that after holding back years of pent-up pain and regret, Schmidt will finally explode and reach some sort of epiphany. A great cathartic release appears to be in store, particularly considering how carefully Nicholson holds it all in. That release comes, briefly, at the film’s end. The small epiphany that tugs a tear from Schmidt’s eye epitomizes the film’s sweet and simple nature.

“About Schmidt,” littered with existential riffs and somber soliloquies, attempts something great. But because the film mirrors its title character, it never achieves greatness. It is a film about that period in life when it is too late to change anything. There is no Hollywood ending here, just the blunt truth.