Charlie Day ("Always Sunny in Philadelphia") and Jenny Slate chatted about their new romantic comedy, "I Want You Back." We put them to the "How Well Do You Know Your Co-Star" test. Watch to find out who knows who better! Day also sang a Whitney Houston/Dolly Parton song to Slate.<br />Read more on SheKnows at: https://www.sheknows.com/entertainment/videos/2535238/jenny-slate-charlie-day-i-want-you-back-exclusive/<br /><br />Video credits<br />VP, Video: Reshma Gopaldas<br />Editor: Emily Collins
