A Doctor of What Exactly?
For a Ghostbuster, Venkman sure doesn't know much about busting ghosts. The truth is that our protagonist is a bad scientist who seems to have only gotten a doctorate in parapsychology because it was an easy way to get a free ride at a university. (Academics everywhere have probably questioned that premise, but anyway...)
Early in the movie, Dr. Peter Venkman's free ride at Columbia is yanked away by Dean Yeager, who totally has Pete's number. Check out this quote where Yeager cuts Peter down to size:
DEAN YEAGER: Dr. Venkman, we believe that the purpose of science is to serve mankind. You, however, seem to regard science as some kind of dodge or hustle. Your theories are the worst kind of popular tripe. Your methods are sloppy, and your conclusions are highly questionable. You, Dr. Venkman, are a poor scientist.
Yeager might be one of the first "bad guys" we meet in the movie, but he's absolutely right about Venkman. When the movie first kicks off, Peter doesn't even believe in ghosts and pokes fun at Ray and Egon's paranormal obsession.
Later, when the ghostbusting business is booming, Pete relies on his buddies' gadgets and scientific knowhow to get the ghosts busted. The real question is if Pete is a total know-nothing, why do Ray and Egon bother to keep around?
When it comes down to it, Ray and Egon need Peter as much as he needs them. While they're good at researching, inventing things, and all that good stuff, Pete is the guy that comes up with the ideas. Like... oh, say... the whole idea of ghostbusting as a business. He's the one that talks them into starting the Ghostbusters with pep talks like this:
PETER: Will you guys relax? We are on the threshold of establishing the indispensable defense science of the next decade. Professional paranormal investigations and eliminations. The franchise rights alone will make us rich beyond our wildest dreams.
So before anybody says that Dr. Venkman is a waste of space, just remember that there'd be no Ghostbusters without his crazy ideas. He might see science as a way to hustle for cash, but it's his way of looking things that actually proves to be the inspiration for the Ghostbusters. If Peter wasn't Peter, all of NYC might've turned into Gozer's permanent playground of destruction.
Venkman also works as the unelected spokesman of the Ghostbusters. In their first business negotiation with the hotel manager after they bag Slimer, it's Peter who sets the price for their services and threatens to release the slimy specter loose again if the manager doesn't pay up. (Ah, nothing like a little extortion to get what you want.) Venkman also has a good old time pumping up the crowd as the Ghostbusters get ready to take down Gozer.
Of course, Peter isn't exactly the most reliable spokesman out there; we probably wouldn't put him on PR team for Shmoop. His biggest failure probably comes when he antagonizes EPA agent Walter Peck for no good reason. In the span of like two minutes, Pete manages to enrage Peck so much that the EPA agent comes back with court orders to shut down the Ghostbusters' containment unit.
Knowing he's gone too far, Venkman tries to apologize, but Peck is so infuriated that he shuts down the unit, causing total chaos in NYC as specters spatter the city. Of course, we can't totally blame Pete for this—Peck is being a whiny wannabe tyrant—but still the whole thing might've been avoided if Venkman knew when to keep his mouth shut.
From the first scene where Pete uses a scam experiment to seduce a young co-ed, it's pretty clear that he's kind of a sleaze. As soon as the beautiful cellist Dana Barrett walks into Ghostbusters HQ for the first time, Dr. Venkman is up to his sleazy tricks again—even leaping over a low wooden wall to hasten the hitting-on-her process.
If he wasn't so oddly lovable, think how creepy some of his first dialogue with Dana would be. Imagine if some supposed professional came over to your house and this was how the conversation went down...
DANA: That's the bedroom, but nothing ever happened in there.
PETER: What a crime.
Cute, Pete. By the end of the "inspection," Dr. Venkman is begging for a kiss and declaring his love for Dana. But Ms. Barrett is an intelligent woman who isn't going to be easily won over by Venkman's... um... charms, and she gives him the boot.
Later, after the Ghostbusters are starting to get famous, Peter redeems himself a bit with Dana by bringing her some info on her Zuul problem (though we know Egon and Ray probably did the work). Somehow, Venkman's combination of weird charm and weirder professional accomplishments win Dana over, and she agrees to a date despite herself.
On date night, Peter manages to shed his sleazy reputation... mostly. He shows up to Dana's apartment and finds her possessed by a horny Zuul, and he actually manages to resist having sex with her. Check it:
DANA: Do you want this body?
PETER: Is this a trick question? I guess the roses worked, huh.
DANA: Take me now, subcreature.
PETER: [...] I make it a rule never to get involved with possessed people. Actually, it's more of a guideline than a rule. [...] No, I can't. Sounds like you've already got at least two people in there.
So even though Venkman does have to wrestle with the decision of whether or not to take advantage of a woman who's possessed by an evil Sumerian demi-god, he does make the right decision in the end. Of course, that doesn't mean he doesn't leave the apartment without giving her a long, weirdo kiss on the neck.
Dr. Peter Venkman: making that subtle creepy vibe fun for the whole family.
For Pete's Sake
Okay, so Venkman is a sleazy sham of a scientist, who likes to run off at the mouth. Then why exactly do we love him so much? First off, despite all his flaws, Venkman does end up being a heroic guy.
When Zuul possesses Dana and Gozer threatens to destroy the city, Peter could make a break for it, but he stands beside the rest of the Ghostbusters in the face of what's more than likely certain death. And when the excrement hits the air conditioning in the final showdown, it's Pete Venkman who rallies the guys with battle cries like, "Let's show this prehistoric b**** how we do things downtown!"
Of course, Ghostbusters wouldn't have been the smash hit that is was if audiences weren't rooting for its man character from the very beginning. Venkman doesn't really turn into a full-fledged hero until the climax, so why do we care about him before then?
Easy. He's funny.
Above anything else, Ghostbusters is a comedy. It doesn't take a genius to figure out that the best way to make an audience to care about a comic hero is for the hero to have them laughing the whole way through. With tragic heroes, we often root for them despite their flaws, but with comic heroes we often root for them because of their flaws.
Not only is Peter funny because all of the weird and witty things he says, he's funny because he keeps doing things that are a really bad idea again and again. Venkman is weirdo slacker with a highly questionable moral character—and we love him for it.