Making a stage kiss look realistic and intimate without crossing a line with your fellow actor is an acting technique that requires finesse. Follow these steps to make it look easy.
Have a frank conversation with your fellow actor. Tell him what you expect in the kissing scene. Be sure to let him know what you would find uncomfortable so that there are some very clear boundaries to work with. Hopefully, if the other actor has a good sense of professionalism, he will hear your concerns and wishes and share some of his own. If you are nervous about having this conversation, try a bit of lighthearted humor to lighten the mood.
Communicate with the director. Find out what her goals are for this portion of the scene. When you truly understand the motivation of the character it is easy to proceed with a stage kiss that is appropriate for the action.
Work with your fellow actor to literally choreograph the stage kiss. A kiss on stage should be handled just like any other physical action scene such as a dance or a fight. Each movement should be planned and rehearsed so that the actors feel comfortable with the kiss. Knowing how your partner will move and where his hands and face will be during the scene eliminates anxiety when in front of an audience.
If you feel uncomfortable during a kissing scene at any time during rehearsals, feel free to stop the scene and address the issue. Use mints or gum prior to your scene and avoid smoking or drinking coffee before the show. This is out of courtesy to your fellow actor.
Never use your tongue when performing a stage kiss. Realistic passionate embraces can be staged without crossing the line and becoming that intimate with a fellow actor.