There's always too much dialogue in spec scripts. If characters are not what they say, but what they do - then guess what, the average character in the average spec simply isn't doing enough.
Of course, many writers attempt to give themselves "get out of jail free" cards when it comes to dialogue. The classics I hear most often are:
1) "This script is for television."
2) "Sitcoms are dialogue-led."
3) "This script is for children."
4) "This script needs a lot of exposition because it's a police procedural/medical series."
5) "This feature is a drama."
So let's take a look at each of them.
1) Television has more dialogue than film, sure. But there aren't many of you out there writing spec soaps -- I get roughly 2 or 3 of those a year at most. Most writers wanting to write for television have spec returning drama series as a sample instead and have exchange after exchange of dialogue, paying TOO MUCH off within it, as if this is the *norm*. It's not. Check out the likes of Spooks, Torchwood, Dr Who, Hustle -- all are known for strong visuals AS WELL AS dialogue. Oh, and enigmatic frontmen who "do" plenty - more than they "say", I would argue.
2) Sitcoms are very much about dialogue, but they're not just about *funny lines*. The sitcoms I read most often could be just gag after gag, concentrating so much on the comedy aspect, the actual situation part is forgotten. What's more, if you watch any of the *great* sitcoms you'll see it's not just about spoken jokes either, but other devices such as reported character, visual gags, farce, structural set ups and pay offs and even a dose of pathos in some of them. So yet again, whilst sitcom MIGHT be "dialogue-led", it's not quite as dialogue-led as you might think... Which is why it's so hard to write.
3) No, no, no... The idea that children need more dialogue than an adult is WRONG. Children growing up now are more media literate than any child of any generation before. They don't need extended chunks of explanation, in fact they're more likely to find THAT confusing! Because to the average kid - especially those under approx 12 - "what you see is what you get", but crucially they're instinctively decoding the layers as well, they *know* there are secret messages. The next generation is all about the visual, don't underestimate that for one second.
4) Police procedurals and medical dramas do need a lot of exposition, sure. But if that exposition is STANDING IN for the actual drama - the *doing*, if you like - then you have a problem, end of.
5) Drama features indulge in the "ordinary" and/or "plausible" and can not only be forgiven for having more dialogue than the genre film, it's ENCOURAGED. But to do this, the dialogue has got to ROCK. Too many writers believe writing a drama means they can write the kind of dialogue THEY might say in REAL LIFE, the type that doesn't really go anywhere or add anything. Actually, in the drama features, its dialogue has to perform the same function as in ALL OTHER scripts: push the story forward, reveal character. Or it needs to be cut.