Here is another technique I learned from Darcy Pattison at the SCBWI Novel Revision Retreat:
Read the first five pages of your manuscript and stop. Now write down everything you know about your protagonist from what you’ve read. Be careful not to include things you, as the author, know. Limit yourself to things only included in these pages.
How did you do? Is your list general and vague, or do you have a strong sense of this person’s personality, preferences, family, community, speech patterns, likes and dislikes, etc.? By five pages, your character should already be defined for your reader.
If you don’t know your character at this point, you might need to move the action forward in your story. Check to see if there is backstory that can be eliminated. Jump in where the story truly begins. Trust your readers to be able to follow.