To get me started in a show it depends on theme, visuals and acting, since those are the things that can show up on trailers and promotional material. Now if the show is already running for sometime word of mouth, reviews or popularity might get me to watch them.
I'm usually really lenient with giving shows a chance (I stuck around the entire first season of Agents of Shield, I'm glad I did, hehehe), but also depends, if there's something I absolutely hate I can stop watching in the first episode.
Now for TV Shows actors don't really draw me in that much. If there's a hot guy I might check it out, but it isn't enough to make me watch it.