Okay. You have an idea. You've either done a plan, or you haven't. Each to their own.
Now, this depends on what type of writer you are. In my limited experience, those that prefer discovery writing will pick up a pen, and start. Maybe not on their book. Maybe they will. Planners won't start yet. Or if they do, it will be character profiles.
But whatever type of writer you are, eventually you're going to come across your characters. You can't have a story without them. It's impossible. Maybe they're not human. Maybe their inanimate dining chairs that want to do ballet. But you will need something.
Of course, if you wish to experiment, be my guest. But generally speaking, a story needs characters.
Some writers will just start. I would recommend that if you're going to do this, start with a character doing something that forces him to react and make a choice. It will tell you, as a writer, about your character. It will give that character a distinct voice from others, as those others make different decisions.
In the end, your characters will be the decisions they make. That's it in a nutshell. If the chair decides to slide along to a ballet class and watch from a corner, this leads to questions. Why isn't he trying it outright? Why did he sneak out of the house? Why hasn't he told his family? If his family disapproves, then why is he still doing it? What inspired his decision?
Every decision has to have a reason behind it, a cause. And every decision will have consequences, will influence the development of your character, change/challange/affirm his moral values. Decision are every thing when it comes to thinking about characters.
After that, everything else is window dressing. What movies they like, what they read, what sports they play - if it matters to the decisions they make, then it is important information. Otherwise, it goes back to being window dressing. Sometimes window dressing becomes important information. The chair likes soccer, he makes a decision to go watch a soccer match, and finds another chair there who wants to do ballet.
Window dressing also differentiates the characters for the reader before the characters start making decisions. While decisions differentiate characters, you can't have major, life changing decisions that show your characters moral compass, religious and cultural identity ever five pages. They will do other things. These other things become important in forging a connection between character and reader.
If you don't start by diving in, you can start with character profiles. You can do a mock interview with your characters. You can chart their entire life history up to this point if that makes you comfortable. If you do plan out who they are before you write, make sure the character fits the type of story you wish to tell. Characters must have motivation for what they're doing - otherwise they become cardboard cutouts. And not many people wish to read about cardboard cutouts.
If you don't plan, my advice would be to start with conflict. It doesn't have to be explosions on the page, it just has to be something that shows us about your character. This start might not nessercarily be part of your book by the time you're finished. some writers start their books and discover their characters as the plot unfolds. There's nothing wrong with that (just be aware there will be editing later). Some writers have to know what their characters will have for breakfast before they can even start page one.
But above all, make your characters memorable. Don't make them perfect (more on this later). Don't make them evil without making them interesting. But make them memorable. I personally will put up with a plot full of holes, as long as I like the characters.