I'm currently writing a script where a woman is in a group that only uses codenames when with each other. I was wondering how I should address this in format. Her real name is known to the audience already so should I keep it her real name when writing the character and actions? I also thought of doing a slash when she's with the group like JONI/WOLFE with "Joni" being her real name and "Wolfe" being her codename. Would that work?
Keep in mind, a script is a technical document intended to aid actors and the director in creating a play or a movie. It is not (directly) for the characters or the audience. The characters might know her as "Wolfe," and the audience might need to get in the mindset of the characters, but what the actors and director most need to know is who is speaking at any given time. So the rule is one actor = one name. (Conversely, if more than one actor plays the same character, each version of that character gets its own name, i.e. YOUNG JONI)
So call her JONI consistently from the start, even if the other characters start exclusively referring to her as "Wolfe." As for the other characters, call them consistently the names they are first introduced by, even if those are codenames.
Pick one character name and stick with it. If you feel the slash name is necessary, use it consistently from the start.
So the rule is generally use the name the audience met the character as first, unless the same actor is being used for two roles or two actors are being used for the same role.
In the first case, the character of Joni and Wolfe are entirely seperate entities, but for thematic reasons, are played by the same actor. This is usually for scenes where there is a dream where Wolfe is the dream version of Joni. Famous examples are Mr. Darling/Captain Hook, from Peter Pan, which has a theatrical tradition of being played by the same actor even though they are not used.
In the former case, the mystery of who Wolfe is may require a second actor to play the character to get a different voice or to throw off the audience until such time as the interloping adolescents (and their canine mascot) pull the mask off to reveal it was Old-Man Jenkins all along. While it is still one character presented in two identities, the demands of the physical script might need a stunt actor to play and need to cue the actor playing the man behind the mask and stunt actor in the actual suit at different times.
Finally, when dealing with the aliases, it's important to ask who is the character really? Among Superhero fans, there is something of a rare agreement that Superman is a created identity for Clark Kent, but Bruce Wayne is the created identity for Batman. This is a notion of "Character is who you are in the dark." or that when a character is all alone and no one (save for the audience) can see what the character's actions, that is when the character truly reveals who they are. Imagine Joni/Wolfe is faced with a sadistic choice where option A would harm Joni's agenda and please Wolfe's agenda and option B would please Joni but harm Wolfe. There are no witnesses or evidence in the story, such that Joni/Wolfe can lie about what happened and no one within the play could possibly impeach her. She and only she would know. The option she takes will reveal who she is.