I've always prided myself on my dialogue. Maybe that's why I put so darn much of it in my stories. Since realistic dialogue is important to me, I hold myself to high standards when writing it.
I'm usually satisfied with how my dialogue sounds after I've taken the all-important step of reading an exchange out loud a few times to determine whether the words could naturally emerge from somebody's mouth. If the lines aren't flowing, I employ some of the useful tricks I've learned, including dropping beginnings of sentences ("Do you need anything at the store?" can become "Need anything at the store?") and adding the nonlinearity that's part of normal conversation.
But I often get hung up in doubt when it comes to the content of what my characters say to each other. I'm deathly afraid of writing dialogue that might come anywhere near the realm of "As you know, Biff, I worked with you at the DMV for thirty years before you embezzled all those vanity plate fees and fled to a country with no extradition treaty." It seems simple enough to avoid writing dialogue that has characters telling each other things they both know. Yet in practice, the problem is often much subtler.
Today I was working on a scene in which some characters get together for a visit. I needed them to discuss one character's job, and it seemed reasonable enough for the topic to come up since they hadn't seen each other in a while. But as soon as I started writing, I began to second-guess myself: "They're talking about this over dinner, but it's so important to the character's life, wouldn't it have come up earlier in the day? And we know the characters talked on the phone recently, so is it really possible they'd have new information to cover in this scene?" I honestly don't think any reader would have questioned the validity of the topic, but I couldn't make myself buy it until I changed things around to a scenario that satisfied me.
In general, I probably spend the majority of my staring-into-space time trying to figure out how my characters can believably start discussing whatever topic I need them to talk about in a particular scene. The topic always belongs in the scene because the characters have something to say about it, but I get stuck on how to bring it up. I may have an outline for their conversation, but I need my characters to behave like real people, who rarely sit down to talk with an agenda in mind.
In the same scene with the update on the character's job, I also had to find a justification to discuss a different character's health, plus include a callback to an earlier scene. It was a complicated maneuver, and there was a lot more staring than writing before I got through the scene. The conversation now all fits together to my satisfaction, and I hope it will appear natural and effortless to the reader.
So that's one page of dialogue completed. And as I said, there's a lot of dialogue in my novel. Now you have some idea of why this is all taking so long.