Be yourself

Be yourself. Being yourself around someone is part of what makes you guys best friends. You are who you are, and your best friend will accept that. Being "fake" could lead you to losing your best friend. Be best friends with someone who you know you can be yourself around, because it isn't worth it trying to be someone you're not so you can be closer to someone.
Don't hold things in. If you feel uncomfortable or have hard feelings toward your best friend, talk about it with them. Make things comfortable, and you'll both go through thick and thin together.