It's a darned sight easier to write a play than to direct one! If characters are awkward, you can shoot 'em.
As a writer, you don't have to deal with egos: "I think I need to stand there" which translates to "I want to do my action on the front of the stage so the world will see me, not half way back where it suits the story better."
You don't have to deal with people who argue with your vision: "But I don't like the way this scene makes me look."
Characters don't hold up rehearsals for twenty minutes at a time, with the whole of the rest of the cast bored and angry, while the argumentative one tries to get things done their way. (Hint: it ain't gonna happen. As director, I can see the whole picture and I have a reason for wanting it done the way I say.)
If I am concentrating on one part of the story and one set of characters, the other characters sit quietly and wait for their turn. Actors begin to chat, mess about - I'm not talking children here, but adults, some in main roles, who make comments they think are witty (they're not), mess around, push and shove at each other. The words Grow Up, come to mind.
And if a character is supposed to know some information, they know it and act accordingly. I am amazed at the number of actors who don't know the full story, don't know the parts that don't involve their character (and what's more couldn't care less about them). Then they are surprised when the action calls for them to react. And that's AFTER a quick read through at the audition stage. Erm, tip: read the play, people. Make sure you know it inside out. It wil help your performance.
The one thing where characters and actors do seem to be the same is their tendency to try and blame someone else. "This scene could have been better but so and so was in the wrong place and I couldn't do my piece properly." On the night, you're going to have to. "He delivered the line wrong, so I got confused." Know your part and be in character enough that you can compensate.
Yesterday, my son (not in the play but sometimes acts) told me something. He is severely dyslexic and has a lot of trouble reading, but always knows all his lines, and all everyone else's lines as well. He told me, he struggles through the whole play, and learns it ALL off by heart. That way, no-one can phase him, he knows what to expect and can roll with it, he never misses a cue. It was a very professional attitude, and I was impressed. If only other actors were willing to put so much in. *sighs*