Cover letters are definitly tough to get started. One place to begin is by stream of consciousness, write down everything you’d really like to say to a potential employer about your skills and experience, then go back and try to put those things into cover-letter-suitable language that will really sell you. Save all of your ideas; they may not all work in one letter, but you could use them later in different letters.
I will generally use the same skeleton cover letter for several applications, tweaking the details as needed; but I also change the skeleton periodically when inspiration strikes or the basic facts have changed.
If I’m writing to someone I’ve worked with before, I usually still keep it formal (addressing them by last name), because you never know who else will be reading your letter as well. But I would try to make more specific connections to how my previous experience with them will help me in another position at the same company.
Some stock phrases:
-In my current role as BLANK I have DONE BLANK
-My (insert wonderful trait), and (2nd wonderful trait) plus (another wonderful trait) make me an excellent match for this job’s requirements.
-During my time (with X theater or in X position), I did X,Y, and Z; this knowledge makes me an adaptable and resourceful BLANK...OR....these skills are readily transferable to a job at your theater
-My proven ability to X,Y,Z, makes me a strong candidate for this position.
-Blank, blank, and blank are among the many traits/skills I can bring to a job with your theater company.
-(Sentence about past experience) these skills will serve me well in a simmillar role at your theater.
-As a stage manager, I’ve been responsible for (insert 3 tough jobs). Hire me as an SM for your theater, and you can expect the same level of (organization and attention to detail) that I’ve demonstrated in the past.
-My previous productions range from (classics/small shows) to (world premiers/casts of thousands).