Is it a halachic problem for a man to watch a woman/girls acting if their not singing?
Watching a woman’s play has two distinct issues to it. The singing is kol isha, and looking at the woman while they are performing is including in the prohibition not staring at women (histaklus b’nashim). This is similar to the issue of not watching women dancing at a wedding etc. Granted, the women are not making exciting movements during a play like they are when dancing, nevertheless watching women is histaklus bnashim.
Gan Naul 13-15, Poskim