It is always very bad form to direct another actor or to otherwise ask another actor to do anything for you.
It doesn’t matter if the director isn’t very good. Or if he seems to have trouble communicating something and you think you know where the disconnect is. It’s not your show. Keep your mouth shut.
You undoubtedly have plenty of work to do on your own role. That’s where your attention should be.
I once watched an actor who wasn’t hired until halfway through rehearsals start directing his scene partner ON HIS VERY FIRST DAY AT REHEARSALS. My jaw nearly hit the floor. The director was gracious and let him speak, and the actor to whom he was speaking was so green that he didn’t realize what a faux pas Mr. Newcomer had made. I probably would have slapped Mr. N. upside the head. (His behavior only confirmed my feeling that he was a bit full of himself, and it will probably always color how I view him.)
I’ve also seen an actor ask another to alter the timing of his entrance, which was prescribed by the text, because the way he was doing it (at the director’s instruction) “is throwing me off.” This was even more egregious because the moment belonged to the entering actor, not Mr. Sensitive (see a future post called “It’s Not Your Scene”).
If you genuinely think you can help the director and feel compelled to do so, do it in private, after rehearsal is over. This gives the director the opportunity to either listen without you undermining his authority in front of the cast or else tell you to mind your own business without making a scene.
Your job is to simply receive and respond to what you get from the other actor, not to demand what he isn’t giving you because he’s not good/clever/insightful enough to give it to you. Even if you’re asking for the right thing.
Even if the other actor is upstaging you in the worst way and you have every right to be upset, don’t challenge him on it directly. Let the director know and let him deal with the problem. He can handle it better than you can. You risk making the other actor hate you on some level (theater people are not always mature and professional), and it will make the experience miserable for you both and affect the play.
This doesn’t mean that there isn’t room for you to make suggestions in the moment that you think might be helpful to the production, but they should always be made to the director, not to the other actors. Phraseology matters, too. “Could we try . . .?”, “I wonder if it might work if we . . .?”, and “Might it be even better if . . .?” leave open the possibility that you’ve just come up with a terrible idea and give the director the ability to turn it down politely. Creativity in a play is a group activity, and you are a full player in that. But you aren’t in charge.
And if you aren’t involved in the scene in question, I don’t care how great the idea you have is. Keep it to yourself and share it with the director privately, after rehearsal. It’s not the last time the scene will be worked on; it can wait. Never butt in with your two cents on someone else’s scene unless the director expressly asks for suggestions from the cast as a whole.
In brief: Don’t make the people you work with look stupid or incompetent.