Conflict drives a story. It's what makes the excitement. It provides the obstacles for your character motivation. It's what makes you care as a reader. Without conflict, you've got nothing but dull words on a page. Who would want to read a story where everything was happy and good? Where everything went the main character's way? It might be nice to live, but not so entertaining to read about.
Without conflict and tension, readers will yawn. It needs to be ongoing and in every scene. There are many types of conflict and the smart writer includes all of them into the page. If you only build in one type, you'll still have a dull story.
Conflict Based on Plot This is the main drive to your manuscript. It's Harry trying to deny Voltemort. It's Frodo and Sam journeying to Mordor to oppose evil. It's Bella falling for a vampire and doing everything to make that work.
It's the driving conflict that moves the story. Without this, well, you haven't got squat. It's what blocks the main character from achieving their motivation/goal. Usually it's the conflict you build the rest of the story around in the planning stages.
But ... plot conflict alone isn't going to get you far. You need much more.
Internal Conflict This is the doubt and worry going on in your main character's head. The feelings of insecurity. The fear that paralyzes. The indecision that makes each major choice the main character faces a dilemma.
Imagine a character so secure in themselves and so decided in their ways that they never have doubts. Then picture a big boring Mary Sue, because that's what has been created. That's not the way humans work. They hesitate. They debate. They feel guilt and nerves.
This kind of conflict can come out in internal thoughts, actions (how the character responds to things), and even with dialogue.
Character arc is all about bringing that character around and fighting through their inner conflict.
Whenever there's a big moment, and indeed, in the SLOW moments of a story, add some internal conflict in your main character and you've increased the tension. Not only that, but you've given the reader a way to connect with that character. (Why Harry worries about death just like me.)
Don't forget the internal conflict.
Conflict between Characters Your character is part of a big group working together for a common cause. They should all just be best friends and braid one another's hair, right? Hold hands and sing together. Never argue. Always be in agreement.
Even allies disagree and a smart author knows this. It's Harry, Ron and Hermione splitting up in the middle of the story. It's Boromir trying to take the ring from Frodo. It's disagreement large and small between allies or enemies or even casual strangers your main character meets.
Every character in your story should have a separate and distinct motivation. Sometimes those motivations are going to clash.
Have a spot where the plot lags and not much is happening? A set up chapter before the big stuff gets going again? Throw in a little of this sort of conflict with another character and you've added enough interest to keep readers going until the big stuff happens.
Physical Non-Plot Conflict Life is full of small annoyances. A train stopped on the tracks. A boo boo on your knee from falling down. A car that won't start at the wrong moment. A broken fingernail on prom night. They don't have anything to do with your story line, but they create conflict (and delay) for your main character. These sorts of occurrences can also add interest to the story and become a side plot.
Any obstacle to the main character can become a test of personality. Testing personality is your job as an author. Make things hard for your main character. Who knows they might step up and become a hero.
So when you are editing and reading over your work, check for conflict. Is there a section where tension is missing? Throw in some of the lesser sorts of conflict to supplement your plot. Put pressure on your main character. Your story will benefit.
Have I forgotten anything? What's your favorite type of conflict to add when things are dull?