It's just a job.
You don't know if they are liberals but you do know that they are hired. Who hired them?

Remember early in the campaign when it was claimed that Trump paid actors to cheer?
Donald Trump Campaign Paid Actors $50 To Cheer On His Announcement » Alex Jones' Infowars: There's a war on for your mind!

I'm guessing some of the actors he paid to cheer were liberals. It's just a job.
Donald Trump Vows to Be 'The Greatest Jobs President God Ever Created' - Breitbart