Dennis Quaid's twins are extras in the diner scene. His daughter is the one who points towards Quaid's character during his health incident.
Dennis Quaid's character was a quarterback in high school. Dennis Quaid [not his character] also played a quarterback in Any Given Sunday (1999). If you notice the picture Bart has of his dad, it's a picture of Quaid in his #19 black jersey from Any Given Sunday.
During the opening scene set in 1985, young Bart is riding his bike through town. He passes a movie theater showing the titles Jaws 3-D (1983) and The Goonies (1985) on the marquee. Dennis Quaid, who plays Bart's father, Arthur, played the lead role of Mike Brody in Jaws 3-D in 1983.
In the movie, Trace Adkins's character tells Bart "Bring the Rain" boys when they got to Nashville for the GMA's. This would be the title of a hit song for the band in 2001.
Before Bart (J. Michael Finley) begins writing the title song, I Can Only Imagine, there is a guitar case in the background that says "I Can Only Imagine" on it.