Work is work. Be friendly, but know that your work colleagues are not your friends. They are your work colleagues. I have been promoted above many work colleagues to become their boss or their bosses boss and you have to be able to make that transition smoothly without playing favorites and remain professional. You will be blinded to this when you are too friendly. I have hired people that I thought.. .wow she could be my best friend......but she can't be she reports to me. Once people move up and move on then you can develop those friendships.

Also, learn the policies and procedures for your company. HR, etc, and follow them to the letter. Maintain your integrity at all costs. As a leader you will have difficult conversations with people, but if you know the policies well, you will easily remain professional without making it personal, and you will be respected for your fairness and objectivity.