If the organization is working, even nominally, don't change anything for a time. There are dynamics as a boss you probably don't fully understand so listen closely and tread lightly as you maintain continuity. If the organization is functioning the people know what they are doing.

Leading an organization that knows you as 'one of the guys' can be difficult. Taking orders from a peer is inflammatory. Some of this can be soft petaled by allowing people to come up with their own best solution. In an organization where people generally know what to do simply asking 'what do you think needs to happen', or 'how did we handle it last time'.

You taking over an organization that is generally going in the right direction. With people who are going to resent an major changes. Imagine driving a car on ice. Make any changes gentle and gradual. If you steer or brake too aggressively it will slide and you lose control.

The organization is functioning and the level of control available is limited so it is ripe for simply working over and around any leader that attempts to work the controls too aggressively for the bonds of confidence and loyalty to bear.

Lean on the sergeants. Bring the individual group leaders, those directly under the boss by flow chart of acclaim, and explain that it was just a matter of luck that you got the nod. Smooth over any potential for jealousy by stating clearly that your not going to make big changes an time soon and that you are depending on their judgment and assistance. Further that if they see you screwing up to pull you aside and explain the situation and help you through the transition. That you will listen.

Stay calm, relax, breath, listen. Don't try to changes things. Your there to manage and limit the damage caused by problems. Not necessarily to eliminate the problem. Many angry people can be placated by just listening and making it clear that your listening and really hearing them. Don't fix what isn't broken. Strive for continuity and allow people to see that you can keep things running as smoothly as the old boss.

Once they see you can handle that you can use that base level of confidence and loyalty to leverage improvements down the road once you have earned your spurs.

Do Not start out with the 'there's a new sheriff in town', 'there are going to be some changes', 'we gonna do things a new way', 'shake things up' speech. Ideally an outside observer should see no changes.