play it straight
To do something in a completely serious, straightforward manner, without any kind of jocosity, foolishness, duplicity, deception, etc. I started dating Samantha because I loved her sense of humor and her chilled out manner, but I wish she would play it straight with me about what she thinks about our relationship. I'm tired of playing it straight; from now on, if there's an easy way to make a buck, I'm going to exploit it!