Terry, on the other hand, is an aimless drifter who's traveled across the country getting in and out of trouble as well as prison. Despite his problems, Sammy still loves her brother who's come back home for a prolonged visit and hopes that he and Rudy might bond in a familial way.

Her attention, however, is split between him and Brian (MATTHEW BRODERICK), her new, perfectionist boss who's driving everyone at the bank crazy. Although she immediately butts heads with him and is considering a recent marriage proposal from her longtime boyfriend, Bob (JON TENNEY), Sammy finds herself oddly drawn to Brian. As she tries to sort out her romantic and professional lives, Sammy must also contend with Terry's inconsistent and unpredictable behavior and his influence on Rudy.

Yet, biology is only a small part of why many siblings are also so different from one another in disposition, behavior and personality traits. Without getting into the whole nature versus nurture argument, suffice it to say that while brothers and sisters can often look and act somewhat alike, far more often than not they seem to have come from different parents if not different parts of the world.

That thought serves as the interesting backdrop of screenwriter turned filmmaker Kenneth Lonergan's directorial debut, "You Can Count on Me." A solidly crafted if decidedly less than extravagantly spectacular picture, this is the sort of film that features strong performances and terrific writing and direction, yet on the surface seems so bland and commonplace that many, if not most moviegoers will probably overlook or ignore it.

All of which is too bad since it's a completely engaging, funny and even heartwarming tale of a polar opposite brother-sister pairing and how their differences separate but also connect them. That the film is so good comes as a bit of a shock considering that Lonergan also wrote "The Adventures of Rocky and Bullwinkle" (as well as "Analyze This"), a film about as completely opposite as one could imagine from this one.

Whether or not Lonergan was simply making a living with that effort until he could helm his own feature, he certainly proves himself with this picture that thankfully demonstrates that one needn't cut their teeth on music videos or employ that type of direction and visual effects and editing to make a stellar, freshman directorial effort.

Reportedly based on a one-act play Lonergan wrote while a member of a Manhattan-based theater company, the film might not have any show-stopping moments, a score filled with pop tunes to sell the soundtrack or a lavish, visual flair. Instead, it makes up for such "deficiencies" with compelling and completely engaging and believable substance. Imagine that.

From moment one, the characters and their situations and dilemmas seem real, and Lonergan and his talented cast manage to make us feel as if we've known these people for a lifetime rather than a few short hours. While the material might not be suitable for all viewers (with the language and adulterous behavior, etc.), it's refreshing to see a film like this come along and restore one's faith in the ability of filmmakers to make terrific movies with great characters and a solid story.

Of course, for a small-scale film like this to work, not only must the writing and direction be strong, but the performances also must be just right and feel honest to the characters. In that regard, Lonergan has hit the mother lode with Laura Linney ("The Truman Show," "Primal Fear"). Playing the single mom who loves her brother despite his problems and presence that upsets her apple cart of life yet ultimately allows her to see things clearly for the first time, Linney delivers such a standpoint performance that it will be a shame if she's not acknowledged come award season time.

Perhaps it's because they've worked together over the years in the theater, but Lonergan also gets a strong performance out of Mark Ruffalo ("Committed," "Ride With the Devil") as the aimless and trouble-prone brother. Although he doesn't play the most likable character, Ruffalo easily manages to make him both interesting and sympathetic.

Fellow longtime Lonergan friend Matthew Broderick ("Election," "Addicted to Love") plays against his stereotypical type as the persnickety and adulterous boss (the type Ferris Bueller would have had a field day with). Meanwhile, the presence of Rory Culkin ("Richie Rich") will probably generate concern that the Culkins are trying to replace the Baldwins and Arquettes as Hollywood's most numerous family, but Rory actually does a decent job playing Rudy, and does so without any of the smirking and wisecracking that ultimately did in his older brother, Macaulay.

Overall, the film - which won the Waldo Salt Screenwriting Award at the 2000 Sundance Film Festival and was co-winner of the Grand Jury Prize - is a fabulous debut from Lonergan and may, like Linney's performance - sneak its way into some bigger award nominations later this year/early next year.

While it won't knock your socks off and easily could have come off as melodramatic in its portrayal of strained family dynamics, it is the type of film that should generate some good word of mouth from those who enjoy solidly constructed pictures with good to great performances. True to its name as far as delivering good filmmaking, "You Can Count on Me" rates as an 8 out of 10.