This may sound obvious, but you'd be amazed just how much confusion there can be around the whole area of roles and responsibilities.

This is often...

...because we have a clear idea in our minds of who should be doing what, and we assume everyone else can see the same picture. 

This lack of a common understanding is highly dangerous.  It can lead to confusion, duplication and - worse still - important things being missed.

And sometimes... 

...it's just down to bad - or no - communication.

I well recall one situation I came across where someone was annoyed with a co-worker who suddenly started being really bossy.  It turned out that she had been promoted to manager but the boss hadn't told anyone!

ACTION POINT

Make sure you get clarity of roles fully understood with the whole team at the start of every project.  

This can't be done on a one-to-one basis.  Everyone on the team needs to know what they and everyone else is responsible for if it's going to work seamlessly.

You should also...

...make it part of the agenda and minutes for each and every meeting.  What are the action points, and who will take responsibility for making them happen, needs to be clearly set out for all to see.

And if you ever... 

...catch yourself assuming people know their roles, it's time to check in with them and make sure your understanding is mutually shared.  

That way, everyone can stay focussed and on task, and everything gets done.