Read between the lines and it becomes clear that Tugg's reason for playing Simple Jack or at least being willing to accept the role, when Rick offered it him, was because Rick himself has a mentally handicapped son. The photo showing them being friends even was from the set of Simple Jack.
Kevin Sandusky gave a You Are Better Than You Think You Are speech to Kirk Lazarus after he had a breakdown telling him that he's the reason that Kevin went into acting in the first place.
When Tugg corrected Alpa Chino when he messed up his name. Kevin just gazed at him amazed that someone had remembered his name.
Tugg being presented with a makeshift Oscar after his forced performance of Simple Jack...
Which turns into a Crowning Moment of Funny when his "son" that gave it to him starts stabbing him in the back at the end of the action sequence.
Kirk's relationship with Tugg. He was the one that presented Tugg with the Oscar for a reason.