linguisticsgirl.com: ... In my mind, grammatical *form**(what a word looks like) is clearly separate from grammatical function (what a word does). From ODO: 2.1. Any of the ways in which a word may be spelled, pronounced, or inflected 4.2. [count noun] A set order of words; a formula:
When Michael J. Fox played 'Johnny B. Goode' as Marty McFly in 'Back to the Future' in 1985, he was just aping Chuck Berry's classic fretwork for the camera. (Along with some Jimi Hendrix, Eddie Van Halen and Pete Townsend guitar moves thrown in for good measure.) Today, Fox can really play the song.
The shooting of Back to the Future, part I, were originally started with Eric Stoltz as Marty McFly. However, the production team decided after some time that Stolz was the right actor for the role anyway.