The idea that sex changes everything between a man and a woman in a formerly platonic relationship is nothing new. It’s crammed down our throats in every televised teen drama and most romantic comedy films. But somehow, writer and director Kevin Smith (“Clerks,” “Dogma”) makes the idea fresh and funny again with his new movie, “Zack and Miri Make a Porno.”

The title is hilariously self-explanatory. Lifelong friends and roommates Zack (Seth Rogen, “Knocked Up,” “Pineapple Express”) and Miri (Elizabeth Banks, “Definitely, Maybe,” “W.”) have spent their lives happily goofing around with each other and not living up to their respective potential. Their slacker ways catch up with them when they suddenly can’t pay the rent and their utilities are shut off. So in order to make some extra dough, Zack and Miri… make a porno. Duh.

They recruit their friend Delaney (Craig Robinson, TV’s “The Office”) to financially back the film and ask former high school classmate Deacon (Jeff Anderson, “Clerks”) to film the production. They hold auditions for other cast members, eventually deciding upon ready-for-anything Lester (Jason Mewes, “Clerks,” “Jay and Silent Bob Strike Back”), the uniquely talented Bubbles (vintage adult film actress Traci Lords), stripper Stacey (actual porn star Katie Morgan), and mild-mannered Barry (Ricky Mabe).

I’d like to again point out the title before I note that this film definitely earns its R-rating, but not necessarily for the scenes where the group is working on their pornographic film. Foul language, dirty one-liners, and a couple of raunchy sight gags make this a movie you might not want to bring your grandmother to see. But if you’re on board (and if you buy a ticket to a movie called “Zack and Miri Make a Porno,” you probably know what you’re in for), get ready for a gut-busting good time.

Naturally, the film incorporates a romantic plot between Zack and Miri. The buildup to the sex scene between the characters is great and when the event finally occurs, it turns out to be pretty emotional. However, after the meaningful moment, Banks’ character becomes really unlikable as she pretends the sex didn’t affect her and makes Zack jump through hoops to win her affections- when it’s completely obvious to everyone in the film and the audience that the two clearly love each other.

The fumbling of the romance is frustrating to watch, but it’s my only problem with the script. The romance may not cause sparks, but the comedic aspect hits its target every time. Every joke is a winner, no matter how dirty. The script deals with sexuality and pornography in a funny and surprisingly reverent way. Though pornography is a controversial subject, the film makes a point of showing each cast member in the movie-within-the-movie displaying a great deal of respect for each other when it comes down to “filming their scenes.”

Rogen plays essentially the same character he plays in every movie, but that’s really just his style of comedy. And in this film, he’s working with Smith, not Judd Apatow. Fortunately, Smith puts his own brand of humor in Rogen’s dialogue and performance, making it seem at least a little new. Banks is little more than cute and funny, which is exactly what she was hired to portray. She does an adequate job, but her assigned role doesn’t exactly make her stand.

The real stars of the cast are the secondary characters, particularly Robinson as put-upon producer Delaney and Justin Long (The Mac Guy, “Accepted”) in a too-brief role as a gay porn star boyfriend of a former high school classmate of Zack and Miri. Robinson’s moments on “The Office” are always the highlight of the episode for me, so it was awesome to see him in such a big role in “Zack and Miri.” He has great comedic timing and interacts with every character in the movie in a unique and amusing way.

Long only pops up twice during the film but both of his scenes were my favorite. At first, his portrayal of a gay stereotype bothered me, but there comes a point where it just became so ridiculous and funny that I couldn’t help but laugh.

Smith finally scores outside his View Askewniverse (all of his films featuring Jay and Silent Bob) with “Zack and Miri Make A Porno.” It’s dirty and disgusting… but also heartwarming and hilarious. Only a writer and director like Smith could pull that off.