Whether Tech. is "rehearsal for them" (the actors) or not (and I disagree with the notion that Tech. is NOT for them as well) encourage them to use every moment onstage as if it were. This is how the cast continues to explore the stage, set, props, costumes, lights etc. (their new working environment) and develop the chops necessary to put on a performance. Any suggestion that they should not be working to capacity when they ARE given the chance to run a scene, song, dance, etc. is undercutting rehearsal time.
Use every rehearsal hour to the fullest, for every member of the company; designers, director, choreographer, technicians, stage management, AND actors. Never waste a second of onstage time by putting into someone's head that it's not valuable work time for them. Actors don't "get into character" just before going onstage each night, they develop the means to do it while in rehearsal. Don't cheat them of this time by telling "them" it isn't "theirs."
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Plus making sure they know (as mentioned above) that it isn't a "rehearsal" for them-it is for the techs, and the techs need them there like the actors need props to practice with (though that particular analogy may not be the best to use...)