Dress up a bit, put on light make up and lighten up. Don't worry about how to act! You're hanging out with a friend. Just coincidentally, he's of the opposite sex. Who knows where it might go. 46+ years ago I married my best friend. (A cliche, yeah, but it's true.) When the edges start to burn off of the passion, it's nice to be with someone that you LIKE. Just be you, that's why he likes you. Hope it works out and have fun!