@galileofigaro further to @TennisParent’s comments on Williams math department, I would add that double majoring is quite common at Williams, even in diverse subjects such as math and theater. If you choose not to double major, you still have the option to participate in theater as performance opportunities are open to all students, not just majors.
You should plan to submit a performance supplement with your application.
Williams has terrific theater facilities and while the Williamstown Theater Festival isn’t directly administered by the college, its influence rubs off.