Complex sentence: The second season saw the introduction of Eric Dane (Dr. Mark Sloan) and Sara Ramirez (Dr. Callie Torres); initially cast as recurring characters, both were given star billing at the opening of the third season.
Split into simpler sentences: The second season marked the introduction of Eric Dane (Dr. Mark Sloan) and Sara Ramirez (Dr. Callie Torres). They were initially cast as recurring characters, but both were given star billing at the opening of the third season.